Julian Date Converter
Turn a Julian date code like 26001 into a calendar date and back, see the day of the year for any date, or convert a date and time to the astronomical Julian Day.
Recent conversions
How to use the Julian date converter
The top card shows today as a five-digit code, the day of the year, and the Julian Day number right now. Pick one of the three tabs below it.
- Julian date (YYDDD): type a code from a label or a form, such as 26001 or 2026001, to get the calendar date, the day of the week, the day of the year, the days left in the year and whether it is a leap year. A five-digit code has only two digits for the year, so choose whether they mean 2000 to 2099 or 1900 to 1999. Below it, pick a calendar date to get its YYDDD and YYYYDDD codes.
- Julian Day (JD, MJD): enter a year, month, day and time in UTC to get the Julian Day with its fraction, the Modified Julian Date, the Julian Day Number and the Unix time. The second box goes the other way: type a JD or an MJD to get the date and time.
- Day of year chart: the day number of every date in a year, one column per month, like a printed Julian calendar. Pick any year and press Print chart for a one-page copy. Today is marked in green.
Your entries are saved in this browser, Undo brings back the previous ones, and Share link sends them to someone else. Nothing is sent to a server. For Unix timestamps, use the epoch converter; to count the days to a date, the days until calculator; for clock times in other cities, the time zone converter.
Two meanings of Julian date
People use "Julian date" for two different things, and the converter keeps them apart in separate tabs.
The ordinal date, the one printed on food packaging and in manufacturing lot codes and in military and logistics paperwork, is just the year and the day of the year. January 1 is day 001 and December 31 is day 365, or 366 in a leap year. It has nothing to do with Julius Caesar's calendar; the name stuck by habit.
The Julian Day used in astronomy is a running count of days since noon UTC on January 1, 4713 BC in the Julian calendar, which is JD 0. Days start at noon, so a fraction of .5 means midnight: the Unix epoch, 1970-01-01 00:00 UTC, is JD 2440587.5, and noon on January 1, 2000 is JD 2451545.0. The Modified Julian Date is MJD = JD - 2400000.5; it starts at midnight, and MJD 0 is 1858-11-17 00:00 UTC.
How a YYDDD code is read
Split the code after the year. In 26001 the first two digits, 26, are the year 2026 and the last three, 001, are the day of the year, so 26001 is January 1, 2026. 24366 is December 31, 2024, because 2024 is a leap year with 366 days; 25366 is not a valid date, since 2025 has only 365. The seven-digit form YYYYDDD writes the whole year: 2026060 is March 1, 2026 (31 days of January, 28 of February, then day 60). Some labels shorten the year to one digit, so read those with the decade you expect.
A year is a leap year when it divides by 4, except century years, which must divide by 400: 2000 was a leap year and 1900 was not. In a leap year every date from March 1 on has a day number one higher than in a common year, which is why the day of year chart changes from year to year.
Calendars and time used for the Julian Day
The tool reads dates as Gregorian by default, extended back before the calendar began (proleptic Gregorian), with astronomical year numbering: year 0 is 1 BC and year -4712 is 4713 BC. Pick Julian calendar for a historical date written in the older calendar; every result shows the same day in both calendars, so it also works as a small Julian calendar converter. The Gregorian calendar began in 1582, when Thursday, October 4 in the Julian calendar was followed by Friday, October 15; countries switched at different times after that. Times are UTC; the tool does not convert to other astronomical time scales.
Keyboard shortcuts
- 1, 2, 3: switch between Julian date, Julian Day and the day of year chart
- T: use today (and the current time for the Julian Day)
- P: print the day of year chart
- ← and → on the chart: previous or next year
- /: jump to the first box of the tab
- U or Ctrl+Z: undo
- F: full screen
Questions
What is today's Julian date?
The top card of the converter always shows it: today's five-digit code (YYDDD), the seven-digit form and the day of the year, taken from your device's date. The Julian Day number next to it is for the current moment in UTC.
How do I convert a Julian date to a regular date?
Type the code in the Julian date (YYDDD) tab. For 5 digits the first two are the year and the last three the day of the year; for 7 digits the first four are the year. The converter shows the calendar date and day of the week, and tells you when a day number does not exist in that year.
What day of the year is it?
Look at the top card, or pick any date in Calendar date to Julian date code. The day of year chart lists every date of a year with its number, ready to print.
What is the difference between JD and MJD?
The Julian Day (JD) counts days from noon UTC on January 1, 4713 BC (Julian calendar). The Modified Julian Date subtracts 2400000.5, which gives smaller numbers and days that start at midnight. JD 2451545.0 is MJD 51544.5.
Why does a Julian Day end in .5 at midnight?
Because each Julian Day starts at noon UTC, so midnight falls halfway through it. Astronomers chose noon so that a night of observing stayed on one date.
More in Dates and time
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