`date()` should support dates before 1000 CE

[1ec5]

The documentation for is_date() says it only works for dates starting in 1000 CE:

To be a date the year must be bigger than 1000, the month if present less or equal 12, the day if present less or equal 31, the hour if present less or equal 24, and the minute and second if present less or equal 60.

For OpenHistoricalMap’s Overpass API instance, almost query ends up needing to filter elements by start_date=* and end_date=* tags expressed in ISO 8601 format. These dates can be as far back in time as necessary, but most OHM software supports at least -9999-01-01 to the end of the present year, if not earlier and later than that. We’d like users to be able to filter dates by date(), which would require the ability to evaluate dates further back in time. The alternative is to rely on the lexicographical sorting property of ISO 8601, but this is error-prone when filtering by end_date=*.

[mmd-osm]

The reason why the year has to be >= 1000 is the following date to string conversion: https://github.com/drolbr/Overpass-API/blob/master/src/overpass_api/statements/string_endomorphisms.cc#L166-L167

Since date comparisons are all string based, lexicographical sorting is relevant even for today's date() queries. I'm not sure if there's an easy way to make the BCE use case work without causing any breakage.

[1ec5]

ISO 8601 requires years between 1 and 1000 to be zero-padded to four digits. If lexicographical sorting is the only concern, then it should be trivial to at least support all CE dates. Padding would even let us support dates back to 10,000 BCE (-9999) without breaking lexicographical sorting, but with the caveat that BCE dates have to be specified to the same precision in order to compare them correctly.