Julian Day Number Converter
Convert any Gregorian calendar date to its astronomical Julian Day Number and back, instantly — runs 100% in your browser with no upload.
🔒 Converted in your browser — nothing is uploaded.
What this tool does
This converter turns any Gregorian calendar date into its Julian Day Number (JDN) and turns any Julian Day Number back into a calendar date. The Julian Day Number is a simple running count of whole days that astronomers use to avoid the messiness of months, leap years, and calendar reforms: day 0 is fixed at noon on 24 November 4714 BC in the proleptic Gregorian calendar, and every day since then just adds one. Pick a date or type a number and the other field updates immediately. A handy checkpoint is the J2000.0 epoch — noon on 1 January 2000 — which has the round Julian Day Number 2451545.
Why the Julian Day Number is useful
Because a JDN is just an integer, the number of days between two events is a plain subtraction, with no need to worry about how many days each month has or whether a leap year sits in between. That makes it the standard timestamp for astronomical observations, satellite tracking, historical chronology, and any software that needs to compare or sort dates spanning many centuries. This tool uses the classic Fliegel–Van Flandern integer algorithm, so the result is exact and identical every time. Everything is computed locally in your browser, so you can convert as many dates as you like without limits.
Frequently asked questions
What is the Julian Day Number for 1 January 2000?
Noon on 1 January 2000 — the J2000.0 astronomical epoch — has Julian Day Number 2451545. This tool returns that value when you enter 2000-01-01, and returns 2000-01-01 when you enter 2451545.
Is the Julian Day Number the same as the Julian calendar?
No. Despite the shared name, the Julian Day Number is a continuous count of days used in astronomy, while the Julian calendar is an older calendar system replaced by the Gregorian calendar in 1582. This tool converts to and from Gregorian calendar dates.
Is my date sent to a server?
No. The date and number you enter never leave your device. Both directions of the conversion are calculated entirely in your browser with JavaScript — nothing is uploaded, logged, or stored.