Julian Date Converter
Convert between calendar dates and Julian Day Numbers.
Select a calendar date and click Convert to get its Julian Day Number.
Disclaimer: The results provided by this tool are estimates for informational purposes only. Actual values may vary. Please verify important calculations independently.
How the Julian Day Number is derived
The conversion from a Gregorian calendar date to a Julian Day Number uses a closed-form integer formula developed for astronomical ephemerides. The algorithm adjusts for the Gregorian calendar's leap year rules, the 1582 calendar reform (when 10 days were skipped to realign with the solar year), and the convention that the astronomical day starts at noon UTC rather than midnight. For January 1, 2000 at noon UTC, the JDN is 2,451,545 — a reference value known as J2000.0 and used as the standard epoch for most modern astronomical calculations.
The noon-based boundary is historical: astronomers chose it so that a single night of observation would carry one JDN rather than span two. Civil systems that prefer midnight-anchored days use the Modified Julian Date (MJD), which subtracts 2,400,000.5 from JDN. That half-day shift moves the day boundary to midnight UTC and yields smaller, more manageable numbers — MJD 60000 corresponds to February 25, 2023. GPS, satellite tracking, and scientific databases commonly store timestamps as MJD plus a fractional day representing UTC hours, minutes, and seconds since the last midnight.
Why continuous day counts matter
The practical attraction of a Julian Date is that it reduces date arithmetic to simple integer subtraction. The number of days between two calendar dates is just the difference of their JDNs, regardless of whether the interval crosses leap years, calendar reforms, or month boundaries. That property makes JDN the canonical internal format for many databases, orbital propagators, and long-term chronological research. Historians comparing events recorded under the Julian calendar (used in Europe until 1582 and in Russia until 1918) with events recorded under the Gregorian calendar can align them unambiguously through a shared Julian date.
Everyday examples illustrate the scale. The Unix epoch (January 1, 1970 at 00:00 UTC) corresponds to JDN 2,440,587.5 or MJD 40587. The first Moon landing on July 20, 1969 sits at JDN 2,440,423. A child born today will reach JDN 2,470,000 sometime in 2080. These large numbers explain why MJD exists for civil and engineering use — five digits is more ergonomic than seven — while full Julian Date remains standard in astronomy because the longer history of recorded observations benefits from a continuous count stretching back to 4713 BC.