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Time Ontology Extended for Non-Gregorian Calendar Applications

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Time Ontology Extended for Non-Gregorian Calendar Applications Editor: Mark Gahegan, University of Auckland, New Zealand Solicited review(s): Karl Grossner, Stanford University, USA; Brandon Whitehead, University of Auckland, New Zealand; one anonymous reviewer Simon J D Cox CSIRO Land and Water, PO Box 56, Highett, Vic. 3190, Australia [email protected] Abstract. We have extended OWL-Time to support the encoding of temporal position in a range of reference systems, in addition to the Gregorian calendar and conventional clock. Two alternative implementations are provided: as a pure extension or OWL-Time, or as a replacement, both of which preserve the same representation for the cases originally supported by OWL-Time. The combination of the generalized temporal position encoding and the tem- poral interval topology from OWL-Time support a range of applications in a variety of cultural and technical set- tings. These are illustrated with examples involving non-Gregorian calendars, Unix-time, and geologic time using both chronometric and stratigraphic timescales. Keywords: temporal reference system, calendar, temporal topology, ontology re-use 1. Introduction spectively). Archaeological and geological applica- tions use chronometric scales based on years counted OWL-Time [6] provides a lightweight model for the backwards from ‘the present’ (defined as 1950 for formalization of temporal objects, based on Allen’s radiocarbon dating [15]), or named periods associat- temporal interval calculus [1]. Developed primarily ed with specified correlation markers [4,5,9]. to support web applications, date-time positions are Since OWL-Time only allows for Gregorian dates expressed using the familiar Gregorian calendar and and times, applications that require other reference conventional clock. systems must look elsewhere. However, the basic Many other calendars and temporal reference sys- structures provided by OWL-Time are not specific to tems are used in particular cultural and scholarly con- a particular reference system, so it would be prefera- texts. For example, the Julian calendar was used ble to use them in the context of a more generic solu- throughout Europe until the 16th century, and is still tion. Some previous extensions to OWL-Time have used for computing key dates in some orthodox been proposed, in particular to deal with aggregates Christian communities. Lunisolar (e.g. Hebrew) and and recurrent intervals [12]. Perrin et al. [13] use the lunar (e.g. Islamic) calendars are still used, and many OWL-Time temporal relations in their geologic time- similar have been used historically. Dynastic calen- scale ontology, but introduce a new class and proper- dars (counting years within eras defined by the reign ty for geochronologic instants. of a monarch or dynasty) were used earlier in many In this paper we describe extensions to OWL-Time, cultures. In more contemporary applications, Loran- which support C, Unix and GPS time are based on seconds counted - the use of an explicit temporal reference system from a specified origin (in 1958, 1970 and 1980, re- when specifying temporal position, - general encodings for dates and times not based 2.2. Temporal position on the Gregorian calendar and clocks. We provide two implementations: “Time-plus” is a Two encodings for temporal position are provided in pure extension to OWL-Time, and “Time-new” is a OWL-Time. Using the datatype xsd:dateTime, posi- generalized replacement 1, which preserves the core tion is encoded as a structured literal, with a preci- of OWL-Time within a modified class hierarchy. sion of seconds or finer indicated by the precision of We illustrate the application of the ontology with a the seconds field in the value. Using the class number of examples. While we do not provide a time:DateTimeDescription temporal position is given model for describing temporal reference systems, we as a value structured using separate properties, with show how the extended ontology supports the de- precision in the range from seconds to years, indicat- 2 scription of ordinal temporal reference systems also ed by the value of the time:unitType property . The extending into deep time. properties time:year, time:month and time:day have range xsd:gYear, xsd:gMonth and xsd:gDay respective- ly, which are tied to the Gregorian calendar [3,14]. 2. Time-plus - extension to OWL-Time Thus, OWL-Time is only capable of expressing tem- poral position in terms of the conventional 12-month 2.1. Temporal reference system calendar with January being the first month of the year, and using the 24-hour clock within each day. A useful classification of temporal reference systems Furthermore, strictly speaking, the Gregorian calen- is provided in ISO 19108 [8]. Four kinds of system dar is valid only for time positions in the ‘Common are distinguished: Era’ (i.e. commencing with the year 1 C.E.). - Coordinate systems, in which a (temporal) po- In order to support a wider range of temporal posi- sition is expressed as a signed quantity, offset tion descriptions, in Time-plus we introduce two from a specified origin classes - tplus:GenDateTimeDescription and - Ordinal reference systems, based on ordered tplus:TimePosition. The former is a generalization named intervals of time:DateTimeDescription, with the range of - Calendars, in which position is expressed us- tplus:year, tplus:month and tplus:day properties ing a year-month-day structure adapted for use with non-Gregorian calendars. In - Clocks, in which position within a day is ex- tplus:TimePosition, two properties provide alterna- pressed in hours-minutes-seconds. tive encodings for the value: tplus:numericValue for The temporal reference system used in OWL-Time position described using a number, and is not explicitly specified [6], but can be inferred to tplus:nominalValue for position indicated as a name. be the Gregorian calendar and conventional clock as The property tplus:hasTRS (described above) is man- this is specified for the xsd:dateTime, xsd:gYear, datory on individuals from both classes, to indicate xsd:gMonth and xsd:gDay datatypes used in encoding the reference system. We also introduce properties values [3,7]. tplus:inDateTime and tplus:inTimePosition as alter- In Time-plus we introduce an additional property, natives to time:inDateTime and time:inXSDDateTime denoted tplus:hasTRS, to support explicit indication provided in OWL-Time. The new classes and proper- of a temporal reference system. Its range is the class ties are summarized in Table 1, with relationships to of temporal reference systems, denoted tplus:TRS. As OWL-Time shown in Figure 1. defined in this ontology, this class is a stub that is These extensions allow temporal position to be only a placeholder for the top of a hierarchy of tem- specified using all the temporal reference system poral reference system types, so no properties are types mentioned in 2.1: defined with the domain tplus:TRS, and there are no - A temporal coordinate using a local restrictions. tplus:TimePosition with a tplus:numericValue - An ordinal position using a tplus:TimePosition with a tplus:nominalValue 1 The Time-plus ontology is published at http://def.seegrid.csiro.au/ontology/time/plus and Time- new at http://def.seegrid.csiro.au/ontology/time/new. Ele- 2 ments of the ontologies are denoted in this paper with the Resources from OWL-Time are denoted in this namespace prefixes tplus: and tnew:, respectively. paper with the prefix “time”. - A position in the Gregorian calendar using necessary. For example, the definition of time:DateTimeDescription with time:unitType set tplus:genMonth is shown in Listing 1, which allows to time:unitDay month values up to 20 in order to accommodate cal- - A position in an arbitrary calendar using endars such as the Hebrew lunisolar calendar (which tplus:GenDateTimeDescription with has a leap-month in some years), the Baha’i calendar time:unitType set to time:unitDay (19 months in a year) and the Mayan calendars (18 - A position in a calendar/clock system using either and 20 periods per year). A note in section 3.3.14 of - time:DateTimeDescription or the XML Schema – Datatypes specification [14] tplus:GenDateTimeDescription with draws attention to the difficulty of converting month time:unitType set to time:unitHour, values between calendars, and warns against use of time:unitMinute or time:unitSecond, gMonth for non-Gregorian calendars. - xsd:dateTime with the default (Gregorian) ref- Note, however, that a generic OWL processor will erence system. not support value-based reasoning over purely lexical The range of tplus:nominalValue in the context of representations. the generalized tplus:TimePosition is a string, since Listing 1 – definition of tplus:genMonth as a string pattern (Turtle the requirement is to support designation of position syntax [2]) in any ordinal reference system, the full set of which tplus:genMonth a rdfs:Datatype ; is not known (e.g. ‘beginning of Late Cambrian’, rdfs:label "Generalized month"^^xsd:string ; ‘end of the bronze age’, ‘end of Ming dynasty’, ‘start owl:onDataType xsd:string ; owl:withRestrictions ( [ xsd:pattern of the reign of Pope Leo IX’). "--(0[1-9]|1[0-9]|20)(Z|(\\+|-)((0[0-9]|1[0-3]):[0- The range of each generalized date property is de- 5][0-9]|14:00))?"^^xsd:string ] ) . fined using the datatype restriction capability of OWL2 [11] so that it has the same lexical form as the original property used in OWL-Time, extended as Table 1 – Elements of the ontology Classes tplus:TRS tplus:GenDateTimeDescription tplus:TimePosition Properties tplus:hasTRS domain tplus:TimePosition|tplus:GenDateTimeDescription range tplus:TRS tplus:inDateTime domain time:Instant range tplus:GenDateTimeDescription domain tplus:GenDateTimeDescription tplus:day range tplus:genDay tplus:month range tplus:genMonth tplus:year range tplus:genYear tplus:inTimePosition range tplus:TimePosition domain tplus:TimePosition tplus:numericValue range tplus:Number tplus:nominalValue range xsd:string Datatypes tplus:genDay String with pattern constraint matching lexical representation of tplus:genMonth xsd:gDay|xsd:gMonth|xsd:gYear tplus:genYear tplus:Number set of numbers Figure 1 – Extended time ontology for supporting additional temporal position encodings.
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