There are many possible valid ISO date formats

[thadguidry]

There are many possible valid ISO date formats. We can add a few more of these also where the dialects have support out-of-the-box, and adjust test response expectations accordingly for each dialect (since many of these tests will fail without EDTF extension support in the database):

• 1999.008 valid year and date in PostgreSQL
• J2451187 valid Julian date in PostgreSQL
• January 8, 99 BC valid year 99 BC in PostgreSQL
• 040506 valid ISO 8601 Time input in PostgreSQL
• 040506+07:30:00 valid UTF offset specified to seconds (not allowed in ISO 8601) but valid Time input in PostgreSQL
• infinity valid special Date/Time input in PostgreSQL which is internally represented as math ∞ but displayed as infinity.
• allballs special Date/Time input in PostgreSQL that represents new day in UTC 00:00:00.00 UTC
• Reduced precision: 2019-12
• Ordinal Date: 2024-336 this depends on the installation config of some DB's but it's generally supported.
• UTC: T144515Z

Repeating intervals might also be stored? but I think this is only special DB modes and like T-SQL.

• R5/2008-03-01T13:00:00Z/P1Y2M10DT2H30M
• P1Y2M10DT2H30M a valid interval in PostgreSQL and others support intervals

  • time, timestamp, and interval accept an optional precision value p which specifies the number of fractional digits retained in the seconds field. By default, there is no explicit bound on precision. The allowed range of p is from 0 to 6.

These are Extended Date Time Formats, but most are not supported in regular modes with the dialects

• 1964/2008 is a time interval with calendar year precision, beginning sometime in 1964 and ending sometime in 2008.
• Y-170000002 is the year -170000002
• 2001-21 is Spring, 2001 (values of 21,22,23,24 signify 'Spring', 'Summer', 'Autumn', 'Winter' and found sometimes in edge network monitoring stations and scientific datasets.
• 2004-06-11% is a year-month-date with uncertainty and approximation.
• 1988/.. open end time interval starting at year 1985 with year precision and open ending.
• /1988 unknown start and ending year 1985.
• 1950S2 represents some year between 1900 and 1999, estimated to be 1950
• 156X-12-25 December 25 sometime during the 1560s.

ERRORS:
-1985 ISO 8601 Part 1 does not support negative years.

Originally posted by @thadguidry in #825 (comment)