Julian Date Converter

A Julian Date counts days continuously since noon UTC on 1 January 4713 BCE, with no months, years, leap years or calendar reform to account for along the way — which is precisely why astronomy uses it. Subtracting two Julian Dates gives the exact number of days between them by simple arithmetic; doing the same with two calendar dates means correctly handling variable month lengths, leap years and, for historical dates, the Julian-to-Gregorian calendar switch.

It starts at noon rather than midnight for a practical, historical reason: 18th-century astronomers observing through the night wanted a single date to cover an entire night's observations rather than having the date roll over partway through, which is exactly what would happen with a midnight-based day.

J2000.0 — Julian Date 2451545.0, exactly noon UTC on 1 January 2000 — is the standard reference epoch modern astronomical position formulas, star catalogues and orbital calculations are built from, this site's own equinox/solstice and moon phase calculations included.