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The GEDCOM X Date Format

Status

This document specifies a date representation for exchanging dates associated with genealogical data, and requests discussion and suggestions for improvements.

The current state of this document is as a "stable draft", and as such the document may be subject to limited changes, BUT NOT backwards-incompatible changes, according to the discussion and suggestions for improvement.

Copyright Notice

Copyright 2011-2013 Intellectual Reserve, Inc.

License

This document is distributed under a Creative Commons Attribution-ShareAlike license. For details, see:

http://creativecommons.org/licenses/by-sa/3.0/

Summary

The GEDCOM X Date Format spec specifies a mechanism for representing dates, especially as they pertain to the need to represent genealogical dates. The spec includes definitions, date ranges, date formats and URI representation examples.

1. Introduction

The GEDCOM X Date specification defines a way of representing dates associated with genealogical data.

This specification is heavily based on the ISO 8601 standard, the RFC 3339 proposal, and W3C's profile of ISO 8601. Concepts from the Dublin Core Date and Time Requirements Wiki were also leveraged.

This specification has been provided because each of these standards or proposals individually has limitations or omissions that do not fulfill the requirements identified for genealogical date representations.

Table Of Contents

1.1 Identifier and Version

The identifier for this specification is:

http://gedcomx.org/date/v1

For convenience, the GEDCOM X date format may be referred to as "GEDCOM X Date 1.0". This specification uses "GEDCOM X Date" internally.

1.2 Notational Conventions

1.2.1 Keywords

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC2119, as scoped to those conformance targets.

2. Terms and Definitions

For the purpose of this document, the following terms and definitions apply in addition to those defined by ISO 8601.

2.1 Basic Terms

2.1.1 calendar date

Portion of a date representing a particular day by specifying its calendar year, its calendar month and the ordinal number of the day within its calendar month.

2.1.2 time of day

Portion remaining from a date if the calendar date portion is ignored, represented in units of hours, minutes, and seconds.

NOTE: By implication, time of day must be less than 24 hours.

2.1.3 CE

Abbreviation of "Common Era", "Current Era", or "Christian Era". Equivalent to "Anno Domini", or "AD".

2.1.4 BCE

Abbreviation for "Before the Common Era", "Before the Current Era", or "Before the Christian Era". The designation "BCE" is to "CE", as "BC" is to "AD".

NOTE: The year preceding 1 CE is identified as "1 BCE". Neither designation uses year 0 (zero).

2.1.5 Gregorian calendar

A calendar introduced in 1582 by Pope Gregory XIII that enhanced the Julian calendar with improved leap year rules.

NOTE: The proleptic Gregorian calendar includes dates prior to 1582 using this calendaring system.

2.1.6 simple date

A date representing a single calendar date, and optionally including a time of day. This term is used to clarify the distinction between a generic date and the aggregation of the GEDCOM X Date types of values (e.g. simple date, date range, open-ended date range, and approximate date).

2.1.7 date range

A time interval can be specified by a start date and an end date (both instances of simple date) or by specifying a start date (a simple date) and a duration. Date ranges MAY be either "closed" (both end points are specified or can be calculated) or "open-ended" (only one end-point is specified).

Examples of closed date range:

  • From January 1863 CE to December 14, 1642 CE

Examples of open-ended date range

  • Until January 1863 CE
  • Since December 14, 1642 CE

2.1.8 recurring date

A series of discrete dates, separated by a specified duration.

Examples:

  • 10 leap years beginning 1924 CE
  • at the same time every day for a week starting at June 18, 1937 CE 10 AM local time
  • every 10 years starting 1820 CE

2.1.9 approximate date

An indeterminate date with a single occurrence roughly centered on a specified simple date.

Examples:

  • About January 1777
  • Around 1590
  • Sometime in 1920

2.1.10 approximate date range

An indeterminate date with a single occurrence within a specified date range.

Examples:

  • Sometime between December 6, 1940 and December 8, 1940

3. Scope

The GEDCOM X Date represents one of the following:

  • a simple date
  • a date range
  • a recurring date
  • an approximate date
  • an approximate date range

3.1 Simple Date

The precision of a simple date is based on the smallest provided unit of measure.

The GEDCOM X Date units of measurement include, and are limited to year, month, day, hour, minute, and second. For a given simple date, all units of measurement larger than the smallest unit specified MUST be provided.

3.2 Date Range

A date range MUST be either a closed date range or an open-ended date range.

3.2.1 Closed Date Range

A closed date range MUST be one of the following:

  • start date and end date
  • start date and duration

3.2.2 Open-Ended Date Range

An open-ended date range MUST include either the start date or the end date, but NOT both.

3.3 Recurring Date

A recurring date is represented by a closed date range providing the following:

  • REQUIRED: a start date (or reference date)
  • REQUIRED: the time interval between occurrences (calculated as the interval between the start date and the end date, or as the interval specified by the duration)
  • OPTIONAL: the number of recurrences

NOTE: If no recurrence count is provided, the recurrences are considered perpetual.

3.4 Approximate Date

An approximate date is represented by providing all of the following:

  • an indicator that the date is approximate
  • a simple date

3.5 Approximate Date Range

An approximate date range is represented by providing all of the following:

  • an indicator that the date is approximate
  • a date range

4. Calendaring System

In order to provide consistency in interpretation of a date, a common calendaring and time system is specified as follows:

  • Dates MUST be specified using the proleptic Gregorian calendar.
  • The earliest representable date is January 1, 10000 BCE.
  • The latest representable date is December 31, 9999 CE.
  • Years are provided as follows:
    • The year prior to 1 CE ("Common Era or "AD") MUST be represented as the year 0.
    • Any year prior to year 0 MUST be represented as a negative number.

5. Format

5.1 Characters Used in Date Representation

The following letters are used as value designators, and precede the value:

  • [A] - designates an approximate date
  • [P] - designates the component as a duration
  • [R] - designates a recurrence count for a recurring date range

The following characters are used as value separators:

  • [T] - separates the calendar date portion of a date or duration from the time of day portion
  • [Z] - designates the date is based on UTC time
  • [-] - separates the values of the calendar date portion's units of a date
  • [:] - separates the values of the time of day portion's units of a date
  • [/] - separates the components of a date range or recurring date range

5.2 Simple Date

5.2.1 Representation

In the format for a simple date, letters are used to represent digits of the date as follows:

  • [Y] - digit used in the year
  • [M] - digit used in the month
  • [D] - digit used in the day of month
  • [h] - digit used in the hour
  • [m] - digit used in the minute
  • [s] - digit used in the second
  • [z] - digit used in the local time offset
  • [±] represents a plus sign [+] if the following element's value is positive or zero, or a minus sign [-] if the following element's value is negative.

The format for a complete simple date is defined as follows:

±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]

5.2.2 Description

The complete simple date format specifies the format of all components and their order (largest to smallest units). Unit components MAY be truncated right-to-left, to indicate precision level of the date.

5.2.2.1 Calendar date part

The year component is defined as a REQUIRED [+] or [-] and four digits, left-padded with zeros as needed. Valid values range from -9999 to +9999. The year component MUST always be present as part of a simple date, and is the maximal unit of precision.

The month component MUST be 2 digits when present, with values of between 01 and 12.

The day of month component MUST be 2 digits when present. The range of valid values is determined by the number of days in that proleptic Gregorian calendar month, with the first day of the month designated as 01.

5.2.2.2 Time of day part

If any time component is present, the character [T] MUST precede the time of day component.

Hours are based on a 24-hour day, and MUST have a value between 00 and 23. In the special case where the minute and second components have zero values, the value 24 is valid, representing midnight at the end of the calendar day. Likewise, if all three components have the value 00, it represents midnight at the beginning of the specified calendar day.

When any time of day is specified, there are three options for specifying its geographical reference:

  • No specifier implies local time
  • [Z] specifies UTC
  • four digits (with a colon separator), preceded by a [+] or [-] indicates the shift of local time from UTC
    • This is usually referred to as the local "time zone"
    • The [+] or [-] character is REQUIRED
    • The first 2 digits represent the hours
    • The last 2 digits represent minutes, and MAY be omitted if zero

5.2.3 Examples

example textual description
+1752-01-18T22:14:3Z January 18, 1752 CE 10:14 and 3 seconds PM UTC
+1964-11-14T10-07:00 November 14, 1964 CE 10 AM, Mountain Standard Time
+1889-05-17T14:23 May 17, 1889 CE 2:23 PM
+1492-07-27 July 27, 1492 CE (presumed to be "local time", honoring the International Date Line)
+0186-03 March 186 CE
-1321 1322 BCE

5.3 Duration

5.3.1 Representation

The initial [P] designates the value is a duration. The part including time components MUST be preceded by [T].

In the format representations for a duration, a digit is represented by the letter [n]. Letters have specific meaning, are literal, and represent the following units:

  • [Y] The number of years
  • [M] The number of months or minutes (determined by context)
  • [D] The number of days
  • [H] The number of hours
  • [S] The number of seconds

The format for a complete duration is defined as follows:

PnnnnYnnMnnDTnnHnnMnnS

5.3.2 Description

A date duration can be represented by a combination of components/units with designators, with the following guidelines and restrictions:

  • Each component is OPTIONAL, and MAY be omitted.
    • If any time component is present, the [T] MUST precede the time of day part.
  • All components present MUST appear in hierarchical order, largest to smallest units.
  • Components are NOT REQUIRED to be normalized.
    • Any non-normalized unit MAY be represented with up to four digits.
    • For example, the descriptive values "13 months" and "2 years, 52 days" each contain non-normalized values and both are considered acceptable.

NOTE: For a duration, local time and UTC distinction is meaningless.

NOTE: A GEDCOM X Date MAY contain a duration, but MUST NOT solely represent a duration itself.

5.3.3 Examples

example textual description
P17Y6M2D duration of 17 years, 6 months, and 2 days
P186D duration of 186 days
PT5H17M lapsed time: 5 hours 17 minutes
P1000Y18M72DT56H10M1S 1000 years 18 months 72 days 56 hours 10 minutes 1 second

5.4 Closed Date Range

5.4.1 Representation

The format for a complete date range is a start date and an end date (both simple dates), separated by a [/]:

±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]/±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]

or a start date (a simple date) and a duration, separated by a [/]:

±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]/PnnnnYnnMnnDTnnHnnMnnS

In either format, the presence of the slash character [/] indicates the date is a date range.

5.4.1.1 Start Date Constraints

The start date (the simple date preceding the slash) MUST NOT be greater than maximum simple date (+9999-12-31T23:59:59) and MUST be earlier than or equivalent to the end date (the simple date following the slash).

NOTE: It is not required that the precision of the two simple dates be the same.

5.4.1.2 Duration Constraints

The duration MUST be such that the calculated end date is earlier or equivalent to the maximum simple date (+9999-12-31T23:59:59).

NOTE: It is not required that the precision of the start date and the duration be the same. The precision of the equivalent end date is the coarser precision of the start date and the duration.

5.4.2 Examples

example textual description
+1752/+1823 from 1752 CE to 1823 CE
+1825-04-13/+1825-11-26 from April 13, 1825 to November 26, 1825
+1933-02-19/P74Y 74 years, starting on February 19, 1933, i.e. from February 19, 1933 to February 19, 2007

5.5 Open-Ended Date Range

5.5.1 Representation

An open-ended date range MUST be a date range where either the start date or end date is explicitly missing.

A leading slash character [/] is used to specify a date range before the provided end date:

/±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]

A trailing slash character [/] is used to specify a date range after the provided start date:

±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]/

5.5.2 Examples

example textual description
/+1887-03 until May, 1887 CE
+1976-07-11/ since July 11, 1976 CE
/-1287 until 1288 BCE
/+0000 until 1 BCE
-0001-04/ since May, 2 BCE

5.6 Recurring Date

5.6.1 Representation

The format for a recurring date is defined as either:

R[n]/±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]/±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]

or

R[n]/±YYYY-MM-DDThh:mm:ss[±hh[:mm]|Z]/PnnnnYnnMnnDTnnHnnMnnS

5.6.2 Description

The recurring date is defined in terms of a closed date range— where start date is the reference date and the recurring interval is calculated as the interval between the start date and the end date or the interval specified by the duration—prepended with an [R], an OPTIONAL recurrence count, and a slash [/].

5.6.3 Examples

example descriptive use case
R4/+1776-04-02/+1776-04-09 every week, for 4 weeks starting on July 2, 1776 CE
R/+2000/P12Y the Chinese Year of the Dragon occurs every 12 years (perpetually), including the year 2000 CE
R100/+1830/+1840 the US census occurs every 10 years starting in 1830, for 100 repetitions

5.7 Approximate Date

The format for an approximate date is defined as a simple date prepended by the character [A].

5.7.1 Examples

example unit of approx textual description
A+1680 year about 1680 CE
A-1400 year about 1401 BCE
A+1980-05-18T18:53Z minutes about 4:53 PM [UTC], May 18, 1980
A+2014-08-19 days about August 19, 2014 CE

5.8 Approximate Date Range

The format for an approximate date range is defined as a date range prepended by the character [A].

5.8.1 Examples

example description, textual equivalent
A+1752/+1823 sometime between 1752 CE and 1823 CE
A+1825-04-13/+1825-11-26 sometime between April 13, 1825 and November 26, 1825
A+1633-02-19/P74Y sometime within 74 years after February 19, 1933
A/+1887-03 sometime before May, 1887 CE
A+1976-07-11/ sometime after July 11, 1976 CE
A/-1287 sometime before 1288 BCE
A/+0000 sometime before 1 BCE
A-0001-04/ sometime before May, 2 BCE

6. URI Representation

A GEDCOM X Date MAY be identified using a Uniform Resource Identifier (URI) as defined by RFC-2396. A URI that identfies a GEDCOM X Date is of the following format:

gedcomx-date:<GEDCOM X Date value>

NOTE: The URI scheme is gedcomx-date and the scheme-specific part is the representation of the date as defined by this specification.

6.1 Examples

example type description applicable URI
simple date Sept 14, 1863 gedcomx-date:+1863-09-14
approx. date about 1742 gedcomx-date:A+1742
date range between October 1834 and May 1835 gedcomx-date:+1834-10/+1835-05

APPENDIX A: Implementation Hints and Observations

The following summaries may be beneficial in parsing and composing GEDCOM X Dates using this specification:

1. Parsing GEDCOM X Dates

  1. Any value that begins with a [+] or a [-] must be a simple date.
    • The [-] will only affect the year component.
    • A negative simple date year component can always be converted to a BCE Gregorian year by adding 1 to the absolute value.
  2. Any value that begins with a [P] must be a duration.
  3. A leading [A] is always an approximate date, and must be followed by either a simple date or a date range.
  4. A leading [R] is always a recurring date range.
  5. A slash [/] always separates values, and its presence always indicates a date range (including open-ended and recurring date ranges).
  6. A [T] always separates the calendar date (calendar units) from the time of day (time units).
  7. Each component of a simple date has a fixed width, always preceded by a designated character in the set [±,-,T,:].
    • All components, except the year component have length of 3, including the delimiting prefix character.
    • The year component has length of 5 (and the prefix is always [+] or [-]).
    • When provided, local time offset has a length of 6, two components (hours and minutes) each of length 3.