A reference,
with its working.

Calendars often print different days for the same festival, and few say why. Software that needs a date has had nothing to check against: no shared statement of the rules, and no common figures to test with. This site is built to be that reference, the way the time-zone database is for clocks.

It gives the same answer to a reader and to a machine: pages, JSON, calendar feeds and an MCP server, from one address per place and day. Where a date is contested it says so, and names who differs. The assumptions are set out under Methods, and the rules, each with its source, under Rules.

The name is the Sanskrit ahargaṇa, the count of days elapsed from an epoch, from which Indian astronomy reckons where the Sun, the Moon and the planets stand.