Enh: simplify date range calculations spanning a month

The date range calculations currently work out the end of the month. This involves leap-year calculations and a table of days in each month.

It would be simpler to express the date range as
gte=YYYY/MM/01 00:00:00
lt=YYYY'/MM'/01 00:00:00
where MM' is MM+1 unless MM=12 etc.

Also, rather than using the string date field, it would likely be more efficient to use the epoch.

Locate the date range calculation described in the issue and inspect how it currently determines month ends and uses the string date field. Compare the proposed month-boundary and epoch-based approach with existing behavior, then verify that date-range results remain correct, including December and leap years.

Written by the indexing model from the issue text.