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

---

• 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

[rettinghaus]

Why reimplement everything from scratch here instead of using https://github.com/ixc/python-edtf?

[rlskoeser]

@rettinghaus thanks for the question. Our underlying data model is different and the goals are different (e.g., supporting different calendars). Also, writing EDTF parsing code based on some of the other parsers out there in various programming languages was a helpful way for me to get more familiar with writing parsers before diving into other formats and calendars.

That said, we should definitely add a link in the readme to python-edtf and clarify where the projects overlap and how they are different.