Julian Date Converter
Convert any date to its day of the year (YYDDD, YYYYDDD, ISO ordinal) or to the astronomical Julian Day Number, Julian Date and Modified Julian Date, convert back the other way, and print a day-of-year calendar for any year.
Day-of-year calendar
How to use the Julian Date Converter
- Pick a date on the Date → Julian tab, or press Today. Add a UTC time if you need an astronomical Julian Date.
- Read the results: day of year, YYDDD, YYYYDDD, ISO ordinal, Julian Day Number, JD and MJD. Use Copy next to any value.
- Convert back: open Julian → Date, choose the input format and type the value, for example 26292 or 2461333.5.
- For YYDDD codes, pick the century if the automatic rule (00–68 = 2000s) is wrong for your data.
- Use the calendar below to look up the day number of any date in the chosen year, and click a day to convert it.
Julian date formats explained
| Format | Example for 19 October 2026 | Where it is used |
|---|---|---|
| YYDDD | 26292 | Food and drug lot codes, military logistics, older mainframe systems |
| YYYYDDD | 2026292 | Mainframes (CYYDDD variants), databases, manufacturing |
| YYYY-DDD (ISO 8601 ordinal) | 2026-292 | Standards, file names, scientific data |
| Julian Day Number | 2461333 | Astronomy, date arithmetic in software |
| Julian Date at 00:00 UTC | 2461332.5 | Astronomy, satellite tracking |
| Modified Julian Date at 00:00 UTC | 61332 | Space science, geodesy, time services |
How the Julian Day Number is calculated
For a Gregorian date with year Y, month M and day D, first set a = ⌊(14 − M) / 12⌋, y = Y + 4800 − a and m = M + 12a − 3. Then:
JDN = D + ⌊(153m + 2) / 5⌋ + 365y + ⌊y / 4⌋ − ⌊y / 100⌋ + ⌊y / 400⌋ − 32045
The Julian Date for a time of day in UTC is JD = JDN − 0.5 + (hours × 3600 + minutes × 60 + seconds) / 86400, because the astronomical day begins at noon. Worked example: 1 March 2024 gives a = 0, y = 6824, m = 0, so JDN = 1 + 0 + 2,490,760 + 1,706 − 68 + 17 − 32,045 = 2,460,371.
Day numbers at the start of each month
| Month | 1st day (common year) | 1st day (leap year) | Month | 1st day (common year) | 1st day (leap year) |
|---|---|---|---|---|---|
| January | 001 | 001 | July | 182 | 183 |
| February | 032 | 032 | August | 213 | 214 |
| March | 060 | 061 | September | 244 | 245 |
| April | 091 | 092 | October | 274 | 275 |
| May | 121 | 122 | November | 305 | 306 |
| June | 152 | 153 | December | 335 | 336 |
To find a day number, add the day of the month minus one to the first-day number. For example 19 October in a common year is 274 + 18 = 292.
Frequently asked questions
What is a Julian date?
The term is used in two ways. In business, food packaging and mainframe systems a Julian date usually means an ordinal date: the year plus the day of the year, such as 26292 or 2026292 for 19 October 2026. In astronomy the Julian Date is a continuous count of days since noon UTC on 1 January 4713 BC (Julian calendar), such as 2461333.0 for noon UTC on 19 October 2026.
How do I read a YYDDD date code?
The first two digits are the year and the last three are the day of the year. 24061 means day 61 of 2024, which is 1 March 2024 because 2024 is a leap year. In a common year day 61 would be 2 March.
What day of the year is it today?
Open the tool and press Today. The large Day of year result shows today's number from 001 to 365, or 366 in a leap year, together with how many days are left in the year.
What is the difference between the Julian Day Number and the Julian Date?
The Julian Day Number (JDN) is a whole number for the day that starts at noon UTC. The Julian Date (JD) adds the fraction of the day, so midnight UTC is JDN − 0.5 and 18:00 UTC is JDN + 0.25. For example 1 January 2000 at 12:00 UTC is JD 2451545.0.
What is the Modified Julian Date?
MJD = JD − 2400000.5. It starts at midnight instead of noon and uses smaller numbers, so midnight UTC on 17 November 1858 is MJD 0 and midnight on 1 January 2000 is MJD 51544.
Does the Julian date have anything to do with the Julian calendar?
Not directly. Ordinal and astronomical Julian dates are counts of days. This converter uses the Gregorian calendar for all dates (extended backwards for dates before 1582), which is the calendar used in everyday life today.
How do leap years change day numbers?
In a leap year 29 February is day 60, so every date from 1 March onwards has a day number one higher than in a common year. 31 December is day 366 in a leap year and day 365 otherwise.