EDTF support - level 0, 1 and parts of level 2

[rlskoeser]

https://www.loc.gov/standards/datetime/

undate supports the date portions of the spec only, not time

• write a code notebook in examples folder and demonstrate support for this; check off items as they are included

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• Level 0
  • Date
    • complete representation: [year][“-”][month][“-”][day]; 1985-04-12’ refers to the calendar date 1985 April 12th with day precision
    • reduced precision for year and month: [year][“-”][month]; ‘1985-04’ refers to the calendar month 1985 April with month precision
    • reduced precision for year: [year]; ‘1985’ refers to the calendar year 1985 with year precision.
  • Time Interval (e.g. 1964/2008, 2004-06/2006-08, 2004-02-01/2005-02-08, 2004-02-01/2005-02)
• Level 1
  • Letter-prefixed calendar year: Y' may be used at the beginning of the date string to signify that the date is a year, when (and only when) the year exceeds four digits, e.g. `Y170000002' is the year 170000002
  • Seasons: values 21, 22, 23, 24 may be used used to signify ' Spring', 'Summer', 'Autumn', 'Winter', respectively, in place of a month value (01 through 12) for a year-and-month format string; 2001-21 = Spring, 2001
  • Qualification of a date (complete) - characters '?', '~' and '%' are used to mean "uncertain", "approximate", and "uncertain" as well as "approximate"; only at end of string
  • Unspecified digit(s) from the right - character 'X' may be used in place of one or more rightmost digits to indicate that the value of that digit is unspecified
  • Extended Interval (L1)
    • null string for unknown start or end date
    • ".." for either start or end date not specified (none or other reasons)
    • modifier at the end of the date to indicate uncertain and/or approximate
  • negative calendar year, e.g. -1985
• Level 2
  • Exponential year, e.g. ‘Y-17E7’
  • Significant digits - year (expressed in any of the three allowable forms: four-digit, 'Y' prefix, or exponential) may be followed by 'S', followed by a positive integer indicating the number of significant digits.
  • Sub-year groupings (extends Level 1 season)
  • Set representation
    • one of a set
    • all members of a set
  • Qualification (uncertain, approximate, both)
    • group qualification
    • individual component
  • Unspecified digit - may occur anywhere
  • Interval - portions of dates may be approximate, uncertain, unspecified