- system for organizing days into repeating cycles aligned to astronomical events
- solar calendars: Gregorian (365.2425 days), Julian, Egyptian
- lunar calendars: Islamic Hijri (354 days), Hebrew
- lunisolar: Chinese, Hindu, Buddhist
- the Gregorian calendar is the global civil standard since 1582
- Unix epoch: seconds since 1970-01-01 00:00:00 UTC, the timekeeping standard of the Information Age
- block height as time in cyber: each block is an irreversible tick of consensus time
- calendars encode cultural, agricultural, and religious rhythms into shared temporal structure
- precondition for coordinated agriculture, trade, navigation, and science