baseDate

A baseDate is a reference timestamp used as the origin from which other times are measured. It provides a fixed point in time that anchors calculations, storage, and comparison of dates in software and data systems. Typically, dates are represented as numeric counts of days, seconds, or milliseconds since the baseDate, enabling efficient arithmetic and simple serialization.

Common baseDate values include the Unix epoch, 1970-01-01 00:00:00 UTC, which is widely used in programming and data interchange. Other systems anchor time to different origins, such as the Windows FILETIME base of 1601-01-01, or spreadsheet software that uses a fixed date as the origin (for example, Excel's 1899-12-30 on Windows and 1904-01-01 on older Mac platforms, with a known leap-year discrepancy for 1900). Julian day counts project time from the ancient epoch of 4713 BCE, another example of a historical baseDate used in astronomical calculations.

Dates can thus be stored as integers or floating-point numbers representing durations since the baseDate, or converted on demand to calendar dates. This approach supports fast comparisons, sorting, and arithmetic like adding a duration or subtracting two dates.

Practical considerations include time zones and daylight saving rules, which affect human-readable representations but may not affect pure time counts. Mismatches in the chosen baseDate across systems can cause off-by-one errors or incorrect era handling, and large counts may risk overflow in limited data types. In software libraries, baseDate may be configurable to accommodate different calendars or eras.

See also: epoch, Unix time, serial date, Julian day.