The civil calendar almost everywhere — and still not the only one a country uses.
The Gregorian calendar is the civil calendar of nearly every state. Introduced in 1582 to correct a drift in the Julian calendar, it fixes the year at 365 days with a leap day in years divisible by four, except centuries not divisible by four hundred.
Being the civil calendar does not make it the only one in use. A great many countries run Gregorian administration alongside a religious or traditional calendar that fixes when the holidays fall — which is why a Gregorian date for Eid, Diwali or Chinese New Year is a conversion rather than a definition, and moves every year.
Every date this corpus publishes is Gregorian, because that is what systems consume. The rule behind it frequently is not, and that is the interesting part.
The Julian calendar is the one Gregorian replaced. Several churches still reckon by it, which is why Orthodox Christmas falls on 7 January in the Gregorian reckoning.
See also julian calendar · leap year · intercalation · lunisolar calendar
We hold this as every observed_date is Gregorian; rule_type records what produced it. See what we hold.