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Julian Date Calculator

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Convert between a calendar date and its Julian day-of-year number (1-365, or 1-366 in leap years).

Julian Date Calculator

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Julian day of year
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This is the day-of-year (ordinal date) format used in manufacturing, military, and expiration codes - not the astronomical Julian Day Number used in astronomy.

The formula

Day of year = number of days elapsed since January 1 of that year, plus 1 (January 1 = day 1, December 31 = day 365, or day 366 in a leap year)
Example

July 19, 2026 is day 200 of 2026. In a leap year like 2024, December 31 is day 366 instead of 365.

Step-by-step guide

  1. Choose your direction - converting a calendar date to a day number, or a day number back to a calendar date.
  2. Enter your date or day number (plus year, if converting from a day number).
  3. Read the result - the tool automatically accounts for leap years.

Why this isn't the astronomical "Julian Date"

Despite the shared name, this tool calculates what's more precisely called an "ordinal date" or "day-of-year" - the format seen on military orders, manufacturing batch codes, and food expiration labels (where a code like "2026200" means day 200 of 2026). The astronomical Julian Date is a different, much larger number representing a continuous day count since 4713 BCE, used in astronomy and some computing contexts - if you're looking for that instead, it requires a different calculation entirely.

Common mistakes

Forgetting leap years - a date after February in a leap year shifts one day later in the count compared to a non-leap year, since February has an extra day.
Confusing this with the astronomical Julian Day Number - despite the same name, they're different systems used for different purposes.

Frequently asked questions

Where are Julian day-of-year codes actually used?

Common uses include military and government date stamps, manufacturing batch codes, and food expiration labels - a canned good stamped "2026045" was packaged on day 45 of 2026, which this calculator shows is February 14.

How do leap years affect the day-of-year number?

Every date from March 1 onward is one day higher in a leap year than the same calendar date in a non-leap year, since February has 29 days instead of 28. December 31 is day 366 in a leap year, day 365 otherwise.

Is this the same as the astronomical Julian Date?

No - despite sharing a name, the astronomical Julian Date is a continuous day count since 4713 BCE used in astronomy, a much larger and differently-calculated number than the day-of-year format this tool provides.

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