zerodatum

Zerodatum is a term used in timekeeping, calendrics, and computational contexts to denote the zero point, or origin, of a date or time scale from which elapsed time is measured. The concept is analogous to an epoch coordinate system, where a specific calendar date and time is designated as the starting reference. A zerodatum is not a universal standard, but rather a chosen reference point suited to a given system. In calendar calculations the zerodatum may coincide with year 1, year 0, or another conventional origin, depending on historical or practical considerations.

In modern computing, several zerodate origins are widely used: for example, the Unix epoch at 1970-01-01 00:00:00 UTC serves as the origin for time values stored as a count of seconds; the GPS epoch at 1980-01-06 marks the start of GPS time; in astronomy the J2000.0 epoch serves as the origin for many coordinate transformations in celestial mechanics. Some software packages designate a different zerodatum for date arithmetic, which can lead to off-by-one or leap-year issues if misinterpreted.

In practice, a zerodatum is a pragmatic choice rather than a physical event. It enables compact storage of elapsed time, straightforward arithmetic, and clear cross-system conversion if documented. Related concepts include epoch, origin, and reference frame.