A DAML Ontology of Time
1. Introduction
A number of sites, DAML contractors and others, have developed
ontologies of time (e.g., DAML-S, Cycorp, CMU, Kestrel, Teknowledge).
A group of us have decided to collaborate to develop a representative
ontology of time for DAML, which could then be used as is or
elaborated on by others needing such an ontology. It is hoped that
this collaboration will result in an ontology that will be adopted
much more widely than any single site's product would be.
We envision three aspects to this effort:
1. An abstract characterization of the concepts and their
properties, expressed in first-order predicate calculus.
2. A translation of the abstract ontology into DAML code, to
whatever extent possible given the current state of DAML
expressivity.
3. Mappings between the DAML ontology and individual sites'
ontologies.
DAML is under development and is thus a moving target, and that is why
separating 1 and 2 is desirable. Level 1 can stabilize before DAML
does. A mapping in 3 may be an isomorphism, or it may be something
more complicated. The reason for 3 is so DAML users can exploit the
wide variety of resources for temporal reasoning that are available.
Moreover, it will aid the widespread use of the ontology if it can be
linked easily to, for example, the temporal portion of Teknowledge's
IEEE Standard Upper Ontology effort or to Cycorp's soon-to-be widely
used knowledge base.
The purposes of the temporal ontology are both for expressing temporal
aspects of the contents of web resources and for expressing
time-related properties of web services.
The following document outlines the principal features of a
representative DAML ontology of time. It is informed by ontology
efforts at a number of sites and reflects but elaborates on a
tentative consensus during discussions at the last DAML meeting. The
first three areas are spelled out in significant detail. The last three
are just sketches of work to be done.
There are a number of places where it is stated that the ontology is
silent about some issue. This is done to avoid controversial choices
in the ontolgy where more than one treatment would be reasonable and
consistent. Often these issues involve identifying a one-dimensional
entity and a zero-dimensional entity with one another.
In general, functions are used where they are total and have a unique
value; predicates are used otherwise. The order of arguments usually
follows the subject-object-object of preposition order in the most
natural use in an English sentence (except for "Hath", where
topicalization applies).
A note on notation: Conjunction (&) takes precedence over
implication(-->) and equivalence (<-->). Formulas are assumed to be
universally quantified on the variables appearing in the antecedent of
the highest-level implication. Thus,
p1(x) & p2(y) --> q1(x,y) & q2(y)
is to be interpreted as
(A x,y)[[p1(x) & p2(y)] --> [q1(x,y) & q2(y)]]
At the end of each section there is a subsection on MAPPINGS. These
are sketches of the relations between some highly developed temporal
ontologies and the one outlined here.
2. Topological Temporal Relations
2.1. Instants and Intervals:
There are two subclasses of temporal-entity: instant and interval.
instant(t) --> temporal-entity(t)
interval(T) --> temporal-entity(T)
(In what follows, lower case t is used for instants, upper case T for
intervals and for temporal-entities unspecified as to subtype. This
is strictly for the reader's convenience, and has no formal
significance.)
beginning-of and end-of are functions from temporal entities to instants.
temporal-entity(T) --> instant(beginning-of(T))
temporal-entity(T) --> instant(end-of(T))
For convenience, we can say that the beginning and end of an instant is
itself.
instant(t) --> beginning-of(t) = t
instant(t) --> end-of(t) = t
inside is a relation between an instant and an interval.
inside(t,T) --> instant(t) & interval(T)
This concept of inside is not intended to include beginnings and ends of
intervals, as will be seen below.
It will be useful in characterizing clock and calendar terms to have a
relation between instants and intervals that says that the instant is
inside or the beginning of the interval.
begins-or-in(t,T) <--> beginning-of(T) = t v inside(t,T)
time-between is a relation among a temporal entity and two
instants.
time-between(T,t1,t2)
--> temporal-entity(T) & instant(t1) & instant(t2)
The two instants are the beginning and end points of the temporal entity.
time-between(T,t1,t2)
<--> beginning-of(T) = t1 & end-of(T) = t2
The ontology is silent about whether the time from t to t, if it
exists, is identical to the instant t.
The ontology is silent about whether intervals _consist of_ instants.
The ontology is silent about whether intervals are uniquely determined
by their beginnings and ends.
We can define a proper-interval as one whose beginning and end are not
identical.
proper-interval(t) <--> interval(t) & beginning-of(t) =/= end-of(t)
The ontology is silent about whether there are any intervals that are
not proper intervals.
2.2. Before:
There is a before relation on temporal entities, which gives
directionality to time. If temporal-entity T1 is before
temporal-entity T2, then the end of T1 is before the beginning of T2.
Thus, before can be considered to be basic to instants and derived for
intervals.
before(T1,T2) <--> before(end-of(T1),beginning-of(T2))
The before relation is anti-reflexive, anti-symmetric and transitive.
before(T1,T2) --> T1 =/= T2
before(T1,T2) --> ~before(T2,T1)
before(T1,T2) & before(T2,T3) --> before(T1,T3)
Negative infinity is before every other instant and positive infinity
is after every other instant.
t =/= *NegInf* --> before(*NegInf*,t)
t =/= *PosInf* --> before(t,*PosInf*)
The end of an interval is not before the beginning of the interval.
interval(T) --> ~before(end-of(T),beginning-of(T))
The beginning of a proper interval is before the end of the interval.
proper-interval(T) --> before(beginning-of(T),end-of(T))
The converse of this is a theorem.
If one instant is before another, there is a time between them.
instant(t1) & instant(t2) & before(t1,t2)
--> (E T) time-between(T,t1,t2)
The ontology is silent about whether there is a time from t to t.
If an instant is inside a proper interval, then the beginning of the
interval is before the instant, which is before the end of the
interval. This is the principal property of "inside".
inside(t,T) --> before(beginning-of(T),t) & before(t,end-of(T))
The converse of this condition is called Convexity and is discussed in
Section 2.4.
The relation "after" is defined in terms of "before".
after(T1,T2) <--> before(T2,T1)
The basic ontology is silent about whether time is linearly ordered.
Thus it supports theories of time, such as the branching futures
theory, which conflate time and possibility or knowledge. This issue
is discussed further in Section 2.4.
The basic ontology is silent about whether time is dense, that is,
whether between any two instants there is a third instant. Thus it
supports theories in which time consists of discrete instants. This
issue is discussed further in Section 2.4.
2.3. Interval Relations:
The relations between intervals defined in Allen's temporal interval
calculus (Allen, 1984; Allen and Kautz, 1985; Allen and Hayes, 1989;
Allen and Ferguson, 1997) can be defined in a straightforward fashion
in terms of before and identity on the beginning and end points.
The
standard interval calculus assumes all intervals are proper, and we
will do that here. We only need to state these for half the relations,
as the other half follow from the definitions below.
int-equals(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-before(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-meets(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-overlaps(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-starts(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-during(T1,T2) --> proper-interval(T1) & proper-interval(T2)
int-finishes(T1,T2) --> proper-interval(T1) & proper-interval(T2)
The definitions of the interval relations in terms of before relations
among their beginning and end points are as follows:
int-equals(T1,T2)
<--> beginning-of(T1) = beginning-of(T2)
& end-of(T1) = end-of(T2)
int-before(T1,T2) <--> before(T1,T2)
int-after(T1,T2) <--> after(T1,T2)
int-meets(T1,T2) <--> end-of(T1) = beginning-of(T2)
int-met-by(T1,T2) <--> int-meets(T2,T1)
int-overlaps(T1,T2)
<--> before(beginning-of(T1),beginning-of(T2))
& before(beginning-of(T2),end-of(T1))
& before(end-of(T1),end-of(T2))
int-overlapped-by(T1,T2) <--> int-overlaps(T2,T1)
int-starts(T1,T2)
<--> beginning-of(T1) = beginning-of(T2)
& before(end-of(T1),end-of(T2)
int-started-by(T1,T2) <--> int-starts(T2,T1)
int-during(T1,T2)
<--> (before(beginning-of(T2),beginning-of(T1))
& before(end-of(T1),end-of(T2))
int-contains(T1,T2) <--> int-during(T2,T1)
int-finishes(T1,T2)
<--> before(beginning-of(T2),beginning-of(T1))
& end-of(T1) = end-of(T2)
int-finished-by(T1,T2) <--> int-finishes(T2,T1)
In addition, it will be useful below to have a single predicate for
"starts or is during". This is called "starts-or-during".
starts-or-during(T1,T2)
<--> [int-starts(T1,T2) v int-during(T1,T2)]
It will also be useful to have a single predicate for intervals
intersecting in at most an instant.
nonoverlap(T1,T2)
<--> [int-before(T1,T2) v int-after(T1,T2) v int-meets(T1,T2)
v int-met-by(T1,T2)]
So far, the concepts and axioms in the ontology of time would be
appropriate for scalar phenomena in general.
2.4. Optional Extensions:
In the basic ontology we have tried to remain neutral with respect to
controversial issues, while producing a consistent and useable
axiomatization. In specific applications one may want to have
stronger properties and thus take a stand on some of these issues. In
this section, we describe some options, with the axioms that would
implement them.
Total or Linear Ordering: In many applications, if not most, it will
be useful to assume that time is linearly or totally ordered. The
axiom that expresses this is as follows:
(A t1,t2)[instant(t1) & instant(t2)
--> [before(t1,t2) v t1 = t2 v before(t1,t2)]]
This eliminates models of time with branching futures and other
conflations of time and possibility.
Infinity: There are two common ways of dealing with the infinite
nature of time. One is to posit time instants at positive and
negative infinity -- *PosInf* and *NegInf*. The principal property of
*PosInf* is that every other instant is before it.
(A t)[instant(t) --> [before(t,*PosInf*) v t = *PosInf*]]
The next axiom says that there are infinitely many instants after any
given instant.
(A t1)[instant(t1)
--> (E t2)[instant(t2) & before(t1,t2)
& before(t2,*PosInf*)]]
The principal property of *NegInf* is that it is before every other
instant.
(A t)[instant(t) --> [t = *NegInf* v before(*NegInf*,t)
The next axiom says that there are infinitely many instants before any
given instant.
(A t1)[instant(t1)
--> (E t2)[instant(t2) & before(t2,t1)
& before(*NegInf*, t2)]]
A postive half-infinite interval T in this approach is one for which
end-of(T) = *PosInf*
and similarly for negative half-infinite intervals.
In the second approach instants at positive and negative infinity are
not posited. Instead axioms state that before and after any instant
there is another instant.
(A t1)[instant(t1) --> (E t2)[instant(t2) & before(t1,t2)]]
(A t1)[instant(t1) --> (E t2)[instant(t2) & before(t2,t1)]]
A positive half-infinite interval T has the following property:
(A t1)[inside(t1,T) --> (E t2)[inside(t2,T) & before(t1,t2)]]
A negative half-infinite interval can be characterized similarly.
Density: In some applications it is useful to have the property of
density, that is, the property that between any two distinct instants
there is a third distinct instant. The axiom for this is as follows:
(A t1,t2)[instant(t1) & instant(t2) & before(t1,t2)
--> (E t)[instant(t) & before(t1,t) & before(t,t2)]]
This is weaker than the mathematical property of continuity, which we
will not axiomatize here. If time is totally ordered and continuous,
it is isomorphic to the real numbers.
Convexity: In Section 2.2 we gave the axiom
inside(t,T) --> before(beginning-of(T),t) & before(t,end-of(T))
The converse of this condition is called Convexity and may be stronger
than some users will want if they are modeling time as a partial
ordering. (See Esoteric Note below.)
before(beginning-of(T),t) & before(t,end-of(T)) --> inside(t,T)
In the rest of this development we will point it out whenever any
concept or property depends on Convexity.
Convexity implies that intervals are contiguous with respect to the
before relation, in that an instant between two other instants inside
an interval is also inside the interval.
before(t1,t2) & before(t2,t3) & inside(t1,T) & inside(t3,T)
--> inside(t2,T)
Extensional Collapse: In the standard development of interval
calculus, it is assumed that any intervals that are int-equals are
identical. That is, intervals are uniquely determined by their
beginning and end points. We can call this the property of
Extensional Collapse.
int-equals(T1,T2) --> T1 = T2
If we think of different intervals between the end points as being
different ways the beginning can lead to the end, then Extensional
Collapse can be seen as collapsing all these into a single "before"
relation.
In the rest of this development we will point it out whenever any
concept or property depends on Extensional Collapse.
Esoteric Note: Convexity, Extensional Collapse, and Total Ordering are
independent properties. This can be seen by considering the following
four models based on directed graphs, where the arcs define the before
relation:
1. An interval is any subset of the paths between two nodes.
(For example, time is partially ordered and an interval is
any path from one node to another.)
2. An interval is the complete set of paths between two nodes.
3. An interval consists of the beginning and end nodes and all the
arcs between the beginning and end nodes but no intermediate
nodes. So inside(t,T) is never true. (This is a hard model
to motivate.)
4. The instants are a set of discrete, linearly ordered nodes.
There are multiple arcs between the nodes. The intervals are
paths from one node to another, including the nodes. (For
example, the instants may be the successive states in the
situation calculus and the intervals sequences of actions
mapping one state into the next. Different actions can have
the same start and end states.)
Model 1 has none of the three properties. Model 2 has Convexity and
Extensional Collapse, but is not Totally Ordered. Model 3 is Totally
Ordered and has Extensional Collapse but not Convexity. Model 4 is
Totally Ordered and Convex, but lacks Extensional Collapse.
2.5. Linking Time and Events:
The time ontology links to other things in the world through four
predicates -- at-time, during, holds, and time-span-of. We assume
that another ontology provides for the description of events -- either
a general ontology of event structure abstractly conceived, or
specific, domain-dependent ontologies for specific domains.
The term "eventuality" will be used to cover events, states,
processes, propositions, states of affairs, and anything else that can
be located with respect to time. The possible natures of
eventualities would be spelled out in the event ontologies. The term
"eventuality" in this document is only an expositional convenience and
has no formal role in the time ontology.
The predicate at-time relates an eventuality to an instant, and is
intended to say that the eventuality holds, obtains, or is taking
place at that time.
at-time(e,t) --> instant(t)
The predicate during relates an eventuality to an interval, and is
intended to say that the eventuality holds, obtains, or is taking
place during that interval.
during(e,T) --> interval(T)
If an eventuality obtains during an interval, it obtains at every
instant inside the interval.
during(e,T) & inside(t,T) --> at-time(e,t)
Whether a particular process is viewed as instantaneous or as occuring
over an interval is a granularity decision that may vary according to
the context of use, and is assumed to be provided by the event
ontology.
Often the eventualities in the event ontology are best thought of as
propositions, and the relation between these and times is most
naturally called "holds". "holds(e,T)" would say that e holds at
instant T or during interval T. The predicate "holds" would be part
of the event ontology, not part of the time ontology, although its
second argument would be be provided by the time ontology. The
designers of the event ontology may or may not want to relate "holds"
to "at-time" and "during" by axioms such as the following:
holds(e,t) & instant(t) <--> at-time(e,t)
holds(e,T) & interval(T) <--> during(e,T)
Similarly, the event ontology may provide other ways of linking events
with times, for example, by including a time parameter in
predications.
p(x,t)
The time ontology provides ways of reasoning about the t's; their use
as arguments of predicates from another domain would be a feature of
the ontology of the other domain.
The predicate time-span-of relates eventualities to instants or
intervals. For contiguous states and processes, it tells the entire
instant or interval for which the state or process obtains or takes
place.
time-span-of(T,e) --> temporal-entity(T)
time-span-of(T,e) & interval(T) --> during(e,T)
time-span-of(t,e) & instant(t) --> at-time(e,t)
time-span-of(T,e) & interval(T) & ~inside(t1,T)
& ~beginning-of(t1,T) & ~end-of(t1,T)
--> ~at-time(e,t1)
time-span-of(t,e) & instant(t) & t1 =/= t --> ~at-time(e,t1)
time-span-of is a predicate rather than a function because until the
time ontology is extended to aggregates of temporal entities, the
function would not be defined for noncontiguous eventualities.
Whether the eventuality obtains at the beginning and end points of its
time span is a matter for the event ontology to specify. The silence
here on this issue is the reason time-span-of is not defined in terms
of necessary and sufficient conditions.
The event ontology could extend temporal functions and predicates to
apply to events in the obvious way, e.g.,
ev-beginning-of(e) = t <--> time-span-of(T,e) & beginning-of(T) = t
This would not be part of the time ontology, but would be consistent
with it.
Different communities have different ways of representing the times
and durations of states and events (processes). In one approach,
states and events can both have durations, and at least events can be
instantaneous. In another approach, events can only be instantaneous
and only states can have durations. In the latter approach, events
that one might consider as having duration (e.g., heating water) are
modeled as a state of the system that is initiated and terminated by
instantaneous events. That is, there is the instantaneous event of
the beginning of the heating at the beginning of an interval, that transitions
the system into a state in which the water is heating. The state
continues until another instantaneous event occurs---the stopping of
the heating at the end of the interval. These two perspectives on
events are straightforwardly interdefinable in terms of the ontology
we have provided. This is a matter for the event ontology to specify.
This time ontology is neutral with respect to the choice.
MAPPINGS:
Teknowledge's SUMO has pretty much the same ontology as presented
here, though the names are slightly different. An instant is a
TimePoint, an interval is a TimeInterval, beginning-of is BeginFn, ans so
on. SUMO implements the Allen calculus.
Cyc has functions #startingPoint and #endingPoint that apply to
intervals, but also to eventualities. Cyc implements the Allen
calculus. Cyc uses a holdIn predicate to relate events to times, but
to other events as well. Cyc defines a very rich set of derived
concepts that are not defined here, but could be.
For instant Kestral uses Time-Point, for interval they use
Time-Interval, for beginning-of they use start-time-point, and so on.
PSL axiomatizes before as a total ordering.
3. Measuring Durations
3.1. Temporal Units:
This development assumes ordinary arithmetic is available.
There are at least two approaches that can be taken toward measuring
intervals. The first is to consider units of time as functions from
Intervals to Reals, e.g.,
minutes: Intervals --> Reals
minutes([5:14,5:17)) = 3
The other approach is to consider temporal units to constitute a set
of entities -- call it TemporalUnits -- and have a single function
_duration_ mapping Intervals x TemporalUnits into the Reals.
duration: Intervals x TemporalUnits --> Reals
duration([5:14,5:17), *Minute*) = 3
The two approaches are interdefinable:
seconds(T) = duration(T,*Second*)
minutes(T) = duration(T,*Minute*)
hours(T) = duration(T,*Hour*)
days(T) = duration(T,*Day*)
weeks(T) = duration(T,*Week*)
months(T) = duration(T,*Month*)
years(T) = duration(T,*Year*)
Ordinarily, the first is more convenient for stating specific facts
about particular units. The second is more convenient for stating
general facts about all units.
The constraints on the arguments of duration are as follows:
duration(T,u) --> proper-interval(T) & temporal-unit(u)
The aritmetic relations among the various units are as follows:
seconds(T) = 60 * minutes(T)
minutes(T) = 60 * hours(T)
hours(T) = 24 * days(T)
days(T) = 7 * weeks(T)
months(T) = 12 * years(T)
The relation between days and months (and, to a lesser extent, years)
will be specified as part of the ontology of clock and calendar below.
On their own, however, month and year are legitimate temporal units.
In this development durations are treated as functions on intervals
and units, and not as first class entities on their own, as in some
approaches. In the latter approach, durations are essentially
equivalence classes of intervals of the same length, and the length of
the duration is the length of the members of the class. The relation
between an approach of this sort (indicated by prefix D-) and the one
presented here is straightforward.
(A T,u,n)[duration(T,u) = n
<--> (E d)[D-duration-of(T) = d & D-duration(d,u) = n]]
At the present level of development of the temporal ontology, this
extra layer of representation seems superfluous. It may be more
compelling, however, when the ontology is extended to deal with the
combined durations of noncontiguous aggregates of intervals.
3.2. Concatenation and Hath:
The multiplicative relations above don't tell the whole story of the
relations among temporal units. Temporal units are _composed of_
smaller temporal units. A larger temporal unit is a concatenation of
smaller temporal units. We will first define a general relation of
concatenation between an interval and a set of smaller intervals.
Then we will introduce a predicate "Hath" that specifies the number of
smaller unit intervals that concatenate to a larger interval.
Concatenation: A proper interval x is a concatenation of a set S of
proper intervals if and only if S covers all of x, and all members of
S are subintervals of x and are mutually disjoint. (The third
conjunct on the right side of <--> is because begins-or-in covers only
beginning-of and inside.)
concatenation(x,S)
<--> proper-interval(x)
& (A z)[begins-or-in(z,x)
--> (E y)[member(y,S) & begins-or-in(z,y)]]
& (A z)[end-of(x) = z
--> (E y)[member(y,S) & end-of(y) = z]
& (A y)[member(y,S)
--> [int-starts(y,x) v int-during(y,x)
v int-finishes(y,x)]]
& (A y1,y2)[member(y1,S) & member(y2,S)
--> [y1=y2 v nonoverlap(y1,y2)]]
The following properties of "concatenation" can be proved as theorems:
There are elements in S that start and finish x:
concatenation(x,S) --> (E! y1)[member(y1,S) & int-starts(y1,x)]
concatenation(x,S) --> (E! y2)[member(y2,S) & int-finishes(y2,x)]
Except for the first and last elements of S, every element of S has
elements that precede and follow it. These theorems depend on the
property of Convexity.
concatenation(x,S)
--> (A y1)[member(y1,S)
--> [int-finishes(y1,x)
v (E! y2)[member(y2,S) & int-meets(y1,y2)]]]
concatenation(x,S)
--> (A y2)[member(y2,S)
--> [int-starts(y2,x)
v (E! y1)[member(y1,S) & int-meets(y1,y2)]]]
The uniqueness (E!) follows from nonoverlap.
Hath: The basic predicate used here for expressing the composition of
larger intervals out of smaller temporal intervals of unit length is
"Hath", from statements like "30 days hath September" and "60 minutes
hath an hour." Its structure is
Hath(N,u,x)
meaning "N proper intervals of duration one unit u hath the proper
interval x." That is, if Hath(N,u,x) holds, then x is the
concatenation of N unit intervals where the unit is u. For example,
if x is some month of September then "Hath(30,*Day*,x)" would be true.
"Hath" is defined as follows:
Hath(N,u,x)
<--> (E S)[card(S) = N
& (A z)[member(z,S) --> duration(z,u) = 1]
& concatenation(x,S)]
That is, x is the concatenation of a set S of N proper intervals of
duration one unit u.
The type constraints on its arguments can be proved as a theorem: N is
an integer (assuming that is the constraint on the value of card), u
is a temporal unit, and x is a proper interval:
Hath(N,u,x) --> integer(N) & temporal-unit(u) & proper-interval(x)
This treatment of concatenation will work for scalar phenomena in
general. This treatment of Hath will work for measurable quantities
in general.
3.3. The Structure of Temporal Units:
We now define predicates true of intervals that are one temporal unit
long. For example, "week" is a predicate true of intervals whose
duration is one week.
second(T) <--> seconds(T) = 1
minute(T) <--> minutes(T) = 1
hour(T) <--> hours(T) = 1
day(T) <--> days(T) = 1
week(T) <--> weeks(T) = 1
month(T) <--> months(T) = 1
year(T) <--> years(T) = 1
We are now in a position to state the relations between successive
temporal units.
minute(T) --> Hath(60,*Second*,T)
hour(T) --> Hath(60,*Minute*,T)
day(T) --> Hath(24,*Hour*,T)
week(T) --> Hath(7,*Day*,T)
year(T) --> Hath(12,*Month*,T)
The relations between months and days are dealt with in Section 4.4.
*****
MAPPINGS:
Teknowledge's SUMO has some facts about the lengths of temporal units
in terms of smaller units.
Cyc reifies durations. Cyc's notion of time covering subsets aims at
the same concept dealt with here with Hath.
Kestrel uses temporal units to specify the granularity of the time
representation.
PSL reifies and axiomatizes durations. PSL includes a treatment of
delays between events. A delay is the interval between the instants
at which two events occur.
4. Clock and Calendar
4.1. Time Zones:
What hour of the day an instant is in is relative to the time zone.
This is also true of minutes, since there are regions in the world,
e.g., central Australia, where the hours are not aligned with GMT
hours, but are, e.g., offset half an hour. Probably seconds are not
relative to the time zone.
Days, weeks, months and years are also relative to the time zone,
since, e.g., 2002 began in the Eastern Standard time zone three hours
before it began in the Pacific Standard time zone. Thus, predications
about all clock and calendar intervals except seconds are relative to
a time zone.
This can be carried to what seems like a ridiculous extreme, but turns
out to yield a very concise treatment. The Common Era (C.E. or A.D.) is
also relative to a time zone, since 2002 years ago, it began three
hours earlier in what is now the Eastern Standard time zone than in
what is now the Pacific Standard time zone. What we think of as the
Common Era is in fact 24 (or more) slightly displaced half-infinite
intervals. (We leave B.C.E. to specialized ontologies.)
The principal functions and predicates will specify a clock or
calendar unit interval to be the nth such unit in a larger interval.
The time zone need not be specified in this predication if it is
already built into the nature of the larger interval. That means that
the time zone only needs to be specified in the largest interval, that
is, the Common Era; that time zone will be inherited by all smaller
intervals. Thus, the Common Era can be considered as a function from
time zones (or "time standards", see below) to intervals.
CE(z) = T
Fortunately, this counterintuitive conceptualization will usually be
invisible and, for example, will not be evident in the most useful
expressions for time, in Section 4.5 below. In fact, the CE
predication functions as a good place to hide considerations of time
zone when they are not relevant. (The BCE era is similarly time zone
dependent, although this will almost never be relevant.)
Esoteric Aside: Strictly speaking, the use of CE as a function depends
on Extensional Collapse. If we don't want to assume that, then we can
use a corresponding predicate -- CEPred(e,z) -- to mean era e is the
Common Era in time zone z.
We have been refering to time _zones_, but in fact it is more
convenient to work in terms of what we might call the "time standard"
that is used in a time zone. That is, it is better to work with *PST*
as a legal entity than with the *PST* zone as a geographical region.
A time standard is a way of computing the time, relative to a
world-wide system of computing time. For each time standard, there is
a zone, or geographical region, and a time of the year in which it is
used for describing local times. Where and when a time standard is
used have to be axiomatized, and this involves interrelating a time
ontology and a geographical ontology. These relations can be quite
complex. Only the entities like *PST* and *EDT*, the time standards,
are part of the _time_ ontology.
If we were to conflate time zones (i.e., geographical regions) and
time standards, it would likely result in problems in several
situations. For example, the Eastern Standard zone and the Eastern
Daylight zone are not identical, since most of Indiana is on Eastern
Standard time all year. The state of Arizona and the Navajo Indian
Reservation, two overlapping geopolitical regions, have different time
standards -- one is Pacific and one is Mountain.
Time standards that seem equivalent, like Eastern Standard and Central
Daylight, should be thought of as separate entities. Whereas they
function the same in the time ontology, they do not function the same
in the ontology that articulates time and geography. For example, it
would be false to say those parts of Indiana shift in April from
Eastern Standard to Central Daylight time.
In this treatment it will be assumed there is a set of entities called
time standards. Some relations among time standards are discussed in
Section 4.5.
4.2. Clock and Calendar Units:
The aim of this section is to explicate the various standard clock and
calendar intervals. A day as a calender interval begins at and
includes midnight and goes until but does not include the next
midnight. By contrast, a day as a duration is any interval that is 24
hours in length. The day as a duration was dealt with in Section 3.
This section deals with the day as a calendar interval.
Including the beginning but not the end of a calendar interval in the
interval may strike some as arbitrary. But we get a cleaner treatment
if, for example, all times of the form 12:xx a.m., including 12:00
a.m. are part of the same hour and day, and all times of the form
10:15:xx, including 10:15:00, are part of the same minute.
It is useful to have three ways of saying the same thing: the clock or
calendar interval y is the nth clock or calendar interval of type u in
a larger interval x. This can be expressed as follows for minutes:
minit(y,n,x)
If the property of Extensional Collapse holds, then y is uniquely
determined by n and x, it can also be expressed as follows:
minitFn(n,x) = y
For stating general properties about clock intervals, it is useful
also to have the following way to express the same thing:
clock-int(y,n,u,x)
This expression says that y is the nth clock interval of type u in x.
For example, the proposition "clock-int(10:03,3,*Minute*,[10:00,11:00))"
holds.
Here u can be a member of the set of clock units, that is, one of
*Second*, *Minute*, or *Hour*.
In addition, there is a calendar unit function with similar structure:
cal-int(y,n,u,x)
This says that y is the nth calendar interval of type u in x. For
example, the proposition "cal-int(12Mar2002,12,*Day*,Mar2002)" holds.
Here u can be one of the calendar units *Day*, *Week*, *Month*, and
*Year*.
The unit *DayOfWeek* will be introduced below in Section 4.3.
The relations among these modes of expression are as follows:
sec(y,n,x) <--> secFn(n,x) = y <--> clock-int(y,n,*sec*,x)
minit(y,n,x) <--> minitFn(n,x) = y <--> clock-int(y,n,*min*,x)
hr(y,n,x) <--> hrFn(n,x) = y <--> clock-int(y,n,*hr*,x)
da(y,n,x) <--> daFn(n,x) = y <--> cal-int(y,n,*da*,x)
mon(y,n,x) <--> monFn(n,x) = y <--> cal-int(y,n,*mon*,x)
yr(y,n,x) <--> yrFn(n,x) = y <--> cal-int(y,n,*yr*,x)
Weeks and months are dealt with separately below.
The am/pm designation of hours is represented by the function hr12.
hr12(y,n,*am*,x) <--> hr(y,n,x)
hr12(y,n,*pm*,x) <--> hr(y,n+12,x)
Each of the calendar intervals is that unit long; a calendar year is a
year long.
sec(y,n,x) --> second(y)
minit(y,n,x) --> minute(y)
hr(y,n,x) --> hour(y)
da(y,n,x) --> day(y)
mon(y,n,x) --> month(y)
yr(y,n,x) --> year(y)
A distinction is made above between clocks and calendars because they
differ in how they number their unit intervals. The first minute of
an hour is labelled with 0; for example, the first minute of the hour
[10:00,11:00) is 10:00. The first day of a month is labelled with 1;
the first day of March is March 1. We number minutes for the number
just completed; we number days for the day we are working on. Thus,
if the larger unit has N smaller units, the argument n in clock-int
runs from 0 to N-1, whereas in cal-int n runs from 1 to N. To state
properties true of both clock and calendar intervals, we can use the
predicate cal-int and relate the two notions with the axiom
cal-int(y,n,u,x) <--> clock-int(y,n-1,u,x)
Note that the Common Era is a calendar interval in this sense, since
it begins with 1 C.E. and not 0 C.E.
The type constraints on the arguments of cal-int are as follows:
cal-int(y,n,u,x) --> interval(y) & integer(n) & temporal-unit(u)
& interval(x)
The temporal units are as follows:
temporal-unit(*sec*), temporal-unit(*min*), temporal-unit(*hr*),
temporal-unit(*da*), temporal-unit(*mon*), temporal-unit(*yr*)
In addition, from below,
temporal-unit(*dayofweek*), temporal-unit(*wk*)
There are properties relating to the labelling of clock and calendar
intervals. If N u's hath x and y is the nth u in x, then n is between
1 and N.
cal-int(y,n,u,x) & Hath(N,u,x) --> 0 < n <= N
There is a 1st small interval, and it starts the large interval.
Hath(N,u,x) --> (E! y) cal-int(y,1,u,x)
Hath(S,N,u,x) & cal-int(y,1,u,x) --> int-starts(y,x)
There is an Nth small interval, and it finishes the large interval.
Hath(N,u,x) --> (E! y) cal-int(y,N,u,x)
Hath(N,u,x) & cal-int(y,N,u,x) --> int-finishes(y,x)
All but the last small interval have a small interval that succeeds
and is met by it.
cal-int(y1,n,u,x) & Hath(N,u,x) & n < N
--> (E! y2)[cal-int(y2,n+1,u,x) & int-meets(y1,y2)]
All but the first small interval have a small interval that precedes
and meets it.
cal-int(y2,n,u,x & Hath(N,u,x) & 1 < n
--> (E! y1)[cal-int(y1,n-1,u,x) & int-meets(y1,y2)]
4.3. Weeks
A week is any seven consecutive days. A calendar week, by contrast,
according to a commonly adopted convention, starts at midnight,
Saturday night, and goes to the next midnight, Saturday night. There
are 52 weeks in a year, but there are not usually 52 calendar weeks in
a year.
Weeks are independent of months and years. However, we can still talk
about the nth week in some larger period of time, e.g., the third week
of the month or the fifth week of the semester. So the same three
modes of representation are appropriate for weeks as well.
wk(y,n,x) <--> wkFn(n,x) = y <--> cal-int(y,n,*wk*,x)
As it happens, the n and x arguments will often be irrelevant.
A calendar week is one week long.
wk(y,n,x) --> week(y)
The day of the week is a temporal unit (*DayOfWeek*) in a larger
interval, so the three modes of representation are appropriate here as
well.
dayofweek(y,n,x) <--> dayofweekFn(n,x) = y
<--> cal-int(y,n,*dayofweek*,x)
Whereas it makes sense to talk about the nth day in a year or the nth
minute in a day or the nth day in a week, it does not really make
sense to talk about the nth day-of-the-week in anything other than a
week. Thus we can restrict the x argument to be a calendar week.
dayofweek(y,n,x) --> (E n1,x1) wk(x,n1,x1)
The days of the week have special names in English.
dayofweek(y,1,x) <--> Sunday(y,x)
dayofweek(y,2,x) <--> Monday(y,x)
dayofweek(y,3,x) <--> Tuesday(y,x)
dayofweek(y,4,x) <--> Wednesday(y,x)
dayofweek(y,5,x) <--> Thursday(y,x)
dayofweek(y,6,x) <--> Friday(y,x)
dayofweek(y,7,x) <--> Saturday(y,x)
For example, Sunday(y,x) says that y is the Sunday of week x.
A day of the week is also a day of the month (and vice versa), and
thus a day long.
(A y)[[(E n,x) dayofweek(y,n,x)] <--> [(E n1,x1) da(y,n1,x1)]]
One correspondance will anchor the cycle of weeks to the rest of the
calendar, for example, saying that January 1, 2002 was the Tuesday of
some week x.
(A z)(E x) Tuesday(dayFn(1,monFn(1,yrFn(2002,CE(z)))),x)
We can define weekdays and weekend days as follows:
weekday(y,x) <--> [Monday(y,x) v Tuesday(y,x) v Wednesday(y,x)
v Thursday(y,x) v Friday(y,x)]
weekendday(y,x) <--> [Saturday(y,x) v Sunday(y,x)]
As before, the use of the functions wkFn and dayofweekFn depend on
Extensional Collapse.
4.4. Months and Years
The months have special names in English.
mon(y,1,x) <--> January(y,x)
mon(y,2,x) <--> February(y,x)
mon(y,3,x) <--> March(y,x)
mon(y,4,x) <--> April(y,x)
mon(y,5,x) <--> May(y,x)
mon(y,6,x) <--> June(y,x)
mon(y,7,x) <--> July(y,x)
mon(y,8,x) <--> August(y,x)
mon(y,9,x) <--> September(y,x)
mon(y,10,x) <--> October(y,x)
mon(y,11,x) <--> November(y,x)
mon(y,12,x) <--> December(y,x)
The number of days in a month have to be spelled out for individual
months.
January(m,y) --> Hath(31,*Day*,m)
March(m,y) --> Hath(31,*Day*,m)
April(m,y) --> Hath(30,*Day*,m)
May(m,y) --> Hath(31,*Day*,m)
June(m,y) --> Hath(30,*Day*,m)
July(m,y) --> Hath(31,*Day*,m)
August(m,y) --> Hath(31,*Day*,m)
September(m,y) --> Hath(30,*Day*,m)
October(m,y) --> Hath(31,*Day*,m)
November(m,y) --> Hath(30,*Day*,m)
December(m,y) --> Hath(31,*Day*,m)
The definition of a leap year is as follows:
(A z)[leap-year(y)
<--> (E n,x)[year(y,n,(CE(z))
& [divides(400,n) v [divides(4,n) & ~divides(100,n)]]]
We leave leap seconds to specialized ontologies.
Now the number of days in February can be specified.
February(m,y) & leap-year(y) --> Hath(29,*Day*,m)
February(m,y) & ~leap-year(y) --> Hath(28,*Day*,m)
A reasonable approach to defining month as a unit of temporal measure
would be to specify that the beginning and end points have to be on the
same days of successive months. The following rather ugly axiom
captures this.
month(T)
<--> (E d1,d2,n,m1,m2,n1,y1,y2,n2,e)
[begins-or-in(beginning-of(T),d1)
& begins-or-in(end-of(T),d2)
& da(d1,n,m1) & mon(m1,n1,y1) & yr(y1,n2,e)
& da(d2,n,m2)
& [mon(m2,n1+1,y1)
v (E y2)[n1=12 & mon(m2,1,y2) & yr(y2,n2+1,e)]]]
The last disjunct takes care of months spaning December and January.
So the month as a measure of duration would be related to days as a
measure of duration only indirectly, mediated by the calendar. It is
possible to prove that months are between 28 and 31 days.
To say that July 4 is a holiday in the United States one could write
(A d,m,y)[da(d,4,m) & July(m,y) --> holiday(d,USA)]
Holidays like Easter can be defined in terms of this ontology coupled
with an ontology of the phases of the moon.
Other calendar systems could be axiomatized similarly. and the BCE
era could also be axiomatized in this framework. These are left as
exercises for interested developers.
4.5. Time Stamps:
Standard notation for times list the year, month, day, hour, minute,
and second. It is useful to define a predication for this.
time-of(t,y,m,d,h,n,s,z)
<--> begins-or-in(t,secFn(s,minFn(n,hrFn(h,daFn(d,
monFn(m,yrFn(y,CE(z))))))))
Alternatively (and not assuming Extensional Collapse),
time-of(t,y,m,d,h,n,s,z)
<--> (E s1,n1,h1,d1,m1,y1,e)
[begins-or-in(t,s1) & sec(s1,s,n1) & min(n1,n,h1)
& hr(h1,h,d1) & da(d1,d,m1) & mon(m1,m,y1)
& yr(y1,y,e) & CEPred(e,z)]
For example, an instant t has the time
5:14:35pm PST, Wednesday, February 6, 2002
if the following properties hold for t:
time-of(t,2002,2,6,17,14,35,*PST*)
(E w,x)[begins-or-in(t,w) & Wednesday(w,x)]
The second line says that t is in the Wednesday w of some week x.
The relations among time zones can be expressed in terms of the
time-of predicate. Two examples are as follows:
h < 8 --> [time-of(t,y,m,d,h,n,s,*GMT*)
<--> time-of(t,y,m,d-1,h+16,n,s,*PST*)]
h >= 8 --> [time-of(t,y,m,d,h,n,s,*GMT*)
<--> time-of(t,y,m,d,h-8,n,s,*PST*)]
time-of(t,y,m,d,h,n,s,*EST*) <--> time-of(t,y,m,d,h,n,s,*CDT*)
The time-of predicate will be convenient for doing temporal
arithmetic.
MAPPINGS:
Teknowledge's SUMO distinguishes between durations (e.g., HourFn) and
clock and calendar intervals (e.g., Hour). Time zones are treated as
geographical regions.
The treatment of dates and times via functions follows Cyc's treatment.
Kestrel's roundabout attempts to state rather straightforward facts
about the clock and calendar are an excellent illustration of the lack
of expressivity in DAML+OIL.
The ISO standard for dates and times can be represented
straightforwardly with the time-of predicate or the unitFn functions.
5. Deictic Time
Deictic temporal concepts, such as ``now'', ``today'', ``tomorrow
night'', and ``last year'', are more common in natural language texts
than they will be in descriptions of Web resources, and for that
reason we are postponing a development of this domain until the first
three are in place. But since most of the content on the Web is in
natural language, ultimately it will be necessary for this ontology to
be developed. It should, as well, mesh well with the annotation
standards used in automatic tagging of text.
We expect that the key concept in this area will be a relation
_now_ between an instant or interval and an utterance or document.
now(t,d)
The concept of "today" would also be relative to a document, and would
be defined as follows:
today(T,d) <--> (E t,n,x)[now(t,d) & begins-or-in(t,T) & da(T,n,x)]
That is, T is today with respect to document d if and only if there is
an instant t in T that is now with respect to the document and T is a
calendar day (and thus the nth calendar day in some interval x).
Present, past and future can be defined in the obvious way in terms of
now and before.
Another feature of a treatment of deictic time would be an
axiomatization of the concepts of last, this, and next on anchored
sequences of temporal entities.
6. Aggregates of Temporal Entities
A number of common expressions and commonly used properties are
properties of sequences of temporal entities. These properties may be
properties of all the elements in the sequence, as in ``every
Wednesday'', or they may be properties of parts of the sequence, as in
``three times a week'' or ``an average of once a year''. We are also
postponing development of this domain until the first three domains
are well in hand.
This may be the proper locus of a duration arithmetic, since we may
want to know the total time an intermittant process is in operation.
A reasonable development of this area would be in terms of the
following subsections.
6.1. Describing Aggregates of Temporal Entities
6.2. Durations as Entities
6.3. Duration Arithmetic
6.4. Rates
7. Vague Temporal Concepts
In natural language a very important class of temporal expressions are
inherently vague. Included in this category are such terms as "soon",
"recently", and "a little while". These require an underlying theory
of vagueness, and in any case are probably not immediately critical
for the Semantic Web. This area will be postponed for a little
while.
References
Allen, J.F. (1984). Towards a general theory of action and time.
Artificial Intelligence 23, pp. 123-154.
Allen, James F., and Henry A. Kautz. 1985. ``A Model of Naive Temporal
Reasoning'', {\it Formal Theories of the Commonsense World}, ed. by
Jerry R. Hobbs and Robert C. Moore, Ablex Publishing Corp., pp. 251-268.
Allen, J.F. and P.J. Hayes (1989). Moments and points in an
interval-based temporal logic. Computational Intelligence 5, pp.
225-238.
Allen, J.F. and G. Ferguson (1997). Actions and events in interval
temporal logic. In Oliveiro Stock (ed.), Spatial and Temporal
Reasoning, Kluwer Academic Publishers, pp. 205-245.