Julian Date Converter
Convert calendar dates to Julian Date, Modified Julian Date and 5-digit YYDDD codes, and back again.
Calendar date → Julian
- Julian Date (JD)
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- Modified Julian Date (MJD)
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- Julian Day Number (JDN)
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- Day of year
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- Packaging code (YYDDD)
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- Ordinal date (YYYYDDD)
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- Day of week
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- Unix timestamp
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Julian → calendar date
Enter a Julian value — the calendar date appears as you type.
- Calendar date (UTC)
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- Time (UTC)
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- ISO 8601
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- Day of week
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- Day of year
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- Julian Date (JD)
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Dates use the proleptic Gregorian calendar (the Gregorian rules extended backwards before 1582), UTC, and the Fliegel & Van Flandern algorithm. Two-digit packaging years are read as 00–69 → 2000–2069 and 70–99 → 1970–1999.
How it works
Astronomers count days instead of juggling months: the Julian Date is the
number of days that have passed since noon UTC on 1 January 4713 BC. Because a Julian Date
rolls over at noon, midnight always lands on a “.5” — 1 January 2000 at
12:00 UTC is exactly JD 2451545.0, and 1 January 1970 at 00:00 UTC is JD 2440587.5. The
Modified Julian Date is simply MJD = JD − 2400000.5, which
starts the count at midnight on 17 November 1858 and keeps the numbers five digits shorter.
This converter uses the integer Fliegel & Van Flandern algorithm in both directions, so
it stays exact for every date from year 1 to year 9999 without relying on floating-point
date libraries.
“Julian date” also means something completely different in factories, kitchens
and warehouses: the YYDDD packaging code stamped on food, medicine and
aviation parts, where 26221 means the 221st day of 2026 — 9 August 2026.
The second panel decodes those five-digit codes as well as seven-digit
YYYYDDD ordinal dates, plain Julian Dates and Modified Julian Dates, and shows
the weekday, day of year and ISO 8601 timestamp for each. Everything is computed in your
browser with plain JavaScript — no uploads, no sign-up and no ads.
Frequently asked questions
What is a Julian Date?
The Julian Date is a continuous count of days since noon UTC on 1 January 4713 BC, used by astronomers so that intervals can be worked out by simple subtraction. Because the count rolls over at noon rather than midnight, civil midnight always lands on a half day: 1 January 2000 at 12:00 UTC is exactly JD 2451545.0, and 1 January 1970 at 00:00 UTC is JD 2440587.5.
What is the difference between JD and MJD?
The Modified Julian Date is just the Julian Date with a shorter, midnight-aligned origin: MJD = JD − 2400000.5. MJD 0 is midnight on 17 November 1858, so MJD values are five digits instead of seven and a whole MJD is a whole civil day. This converter shows both for every date you enter.
How do I read a 5-digit Julian date code on a package?
A YYDDD packaging code is the last two digits of the year followed by the three-digit day of the year, so 26221 means the 221st day of 2026, which is 9 August 2026. Choose "Packaging code (YYDDD)" in the second panel and the tool decodes it, including the weekday. Two-digit years 00 to 69 are read as 2000 to 2069 and 70 to 99 as 1970 to 1999.