Donaldson R F Julian Date Converter
Convert between a calendar date and time, the astronomical Julian Date (JD), the Modified Julian Date (MJD) and a Unix timestamp — in either direction.
What these numbers mean
The Julian Date is a continuous day count used in astronomy: the number of days (and fraction of a day) since noon UTC on 1 January 4713 BC (proleptic Julian calendar). Because JD changes at noon rather than midnight, and quickly becomes a large number, astronomers often use the Modified Julian Date instead, defined as MJD = JD − 2,400,000.5, which starts at midnight and uses smaller numbers. A Unix timestamp is the everyday computing equivalent: seconds since midnight UTC on 1 January 1970.
Frequently asked questions
Why does JD change at noon?
The convention dates back to when astronomers observed at night and did not want a single night's observations to fall across two different day numbers; noon was chosen as the boundary instead of midnight.
Does this handle dates before 1582?
It calculates using the modern (Gregorian) calendar projected backwards, which is what almost all Julian Date calculators do, rather than switching to the older Julian calendar before its adoption in October 1582.
Related tools: Timestamp Converter, Light Travel Time Calculator, Date Calculator.
Source / Last verified: US Naval Observatory, "Converting Between Julian Dates and Gregorian Calendar Dates" (aa.usno.navy.mil/faq/JD_formula), which confirms Julian Day Number 2,440,588 for 1 January 1970 — read and used to check this page's conversion (and its reverse) on 27 September 2026. The Modified Julian Date offset (−2,400,000.5) is the standard definition used throughout astronomy.