Yes. There are two classes of calendar: observational and arithmetic. Modern calendars tend to be arithmetic, based on fixed rules that approximate astronomy m

Yes. There are two classes of calendar: observational and arithmetic. Modern calendars tend to be arithmetic, based on fixed rules that approximate astronomy more or less accurately. The Julian calendar is one of the simplest; the Jewish calendar is one of the most complicated and most accurate. Observational calendars are more directly based on astronomical observations. The Muslim calendar is one example: in some traditions, the start of Ramadan can’t be predicted exactly because it depends on seeing the new moon. UTC is also an observational calendar, since leap seconds are scheduled by observation not by fixed rules. The French revolutionary calendar was also (in its strict form) observational, keeping its years precisely in sync with the equinoxes.