Julian Day Converter
Convert any Gregorian date to its Julian Day Number plus weekday — the astronomer’s day count.
Convert calendar dates to Julian Day Numbers instantly. Free JDN converter with weekday for astronomy and date math.
Calculator Inputs
About Julian Day
Convert any Gregorian date to its Julian Day Number plus weekday — the astronomer’s day count.
Julian Day Numbers count days continuously from noon, January 1, 4713 BC — no months, leap rules, or calendar reforms to trip over. Subtracting two JDNs always gives the exact day difference, which is why astronomers and date libraries use them internally.
Note the noon epoch: a JDN flips at midday UTC, not midnight. Results assume the proleptic Gregorian calendar (extended backward). Everything runs in your browser.
Where it is used
Date-difference math, astronomy logging, verifying weekday claims, and Julian-date data pipelines.
How to use the Julian Day
- Gregorian Date — enter the value.
- Calculate — press the Calculate button to see the result instantly above.
Formula
Fliegel–Van Flandern: JDN from Y/M/D with century corrections; weekday = (JDN+1) mod 7.
Examples
Example
Frequently Asked Questions
- Why does the day start at noon?
- So an entire night of observation falls on one date. Astronomers proposed it in 1849; the convention stuck.
- JDN vs Julian calendar?
- Unrelated namesakes. The Julian Day Number is a continuous count; the Julian calendar is Julius Caesar’s leap-year system with its 11-minute drift.
- How do I get days between two dates?
- Convert both and subtract. Every unit is exactly one day — no month-length tables needed.