AuroraMeter.Period.Calendar (Aurora Meter v1.0.0)

Default period source: calendar month in UTC.

The interval is half-open by construction: the first of the month at 00:00:00Z up to, and not including, the first of the next month at 00:00:00Z. It is a pure function of the instant, so it answers a past instant correctly through AuroraMeter.Period.containing/2's fallback and needs no containing/2 of its own.