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Julian Time Converter: How to Use It

Updated · Standard formula, shown below

The calculator for this page is switched off while it is checked. The formula is below, and the worked example in the article shows how to use it by hand.

A Julian time converter translates between calendar dates and Julian Day Numbers (JDN) or Julian dates, which count days continuously since noon on January 1, 4713 BCE. Enter a date and time (or just a date) and you get the corresponding Julian Day Number, often with the fractional day for the time of day. You can also go the other way, turning a Julian Day Number back into a Gregorian calendar date.

Formula

The standard formula for converting a Gregorian date to a Julian Day Number is:

JDN = (1461 × (Y + 4800 + (M − 14) / 12)) / 4 + (367 × (M − 2 − 12 × ((M − 14) / 12))) / 12 − (3 × ((Y + 4900 + (M − 14) / 12) / 100)) / 4 + D − 32075

Where:

  • Y is the year (e.g., 2025)
  • M is the month (1–12)
  • D is the day of the month (1–31)

All divisions are integer divisions (the fractional part is dropped). The result is an integer that represents the Julian Day Number at noon UTC on that date. For times other than noon, add the fraction of the day: (hour + minute/60)/24. Subtract 0.5 if you want the Julian Day Number to start at midnight instead of noon (the traditional astronomical definition starts at noon).

Worked Example

Let’s convert June 15, 2025, at 12:00 UTC to a Julian Day Number using the default values from the calculator.

  1. Set Y = 2025, M = 6, D = 15.
  2. Compute the first term: (1461 × (2025 + 4800 + (6 − 14)/12)) / 4. (6−14)/12 = −0.666…, integer division gives 0, so (2025+4800+0) = 6825. 1461 × 6825 = 9,969,825. Divide by 4: 2,492,456.25, integer part 2,492,456.
  3. Second term: (367 × (6 − 2 − 12 × ((6−14)/12))) / 12. (6−14)/12 = 0 (integer), so 6−2−0 = 4. 367 × 4 = 1,468. Divide by 12: 122.333, integer part 122.
  4. Third term: (3 × ((2025 + 4900 + (6−14)/12) / 100)) / 4. (2025+4900+0) = 6925. 6925/100 = 69.25, integer 69. 3 × 69 = 207. Divide by 4: 51.75, integer 51.
  5. Now combine: 2,492,456 + 122 − 51 + 15 − 32075 = 2,460,467.
  6. Add the time fraction: 12:00 is 12/24 = 0.5. So JDN = 2,460,467 + 0.5 − 0.5 = 2,460,467.0.

The calculator gives 2460467.0 for June 15, 2025 at 12:00, matching our manual calculation.

Why Use Julian Dates?

Julian Day Numbers were introduced by Joseph Scaliger in 1583 to provide a single continuous count of days that is independent of calendars, months, and years. This makes them extremely useful for:

  • Calculating the number of days between two dates (just subtract the JDN values).
  • Astronomy, where observations are timestamped with a continuous day count.
  • Historical research, where dates from different calendars (Julian vs. Gregorian) need to be compared.
  • Computer systems that need a simple numeric representation of a date.

How to Use the Converter

Using a Julian time converter is straightforward:

  1. Enter the date in the format YYYY-MM-DD (e.g., 2025-06-15).
  2. Enter the time in 24-hour format (e.g., 14:30 for 2:30 PM). The time is optional; if omitted, noon is assumed.
  3. Click convert. The tool returns the Julian Day Number, including the fractional part for the time.

If you need to convert a Julian Day Number back to a date, use the reverse formula or a calculator that does the inverse. The reverse calculation is more complex because it involves determining the year, month, and day from the JDN.

Common Mistakes and Misconceptions

One common error is confusing Julian Day Number (JDN) with Julian Date (JD). JDN is the integer part (the day count), while JD includes the fractional day. For example, at noon on June 15, 2025, the JDN is 2460467, and the JD is 2460467.0. At 6:00 PM, the JD would be 2460467.25.

Another mistake is mixing up the Julian calendar (introduced by Julius Caesar) with Julian Day Numbers. The Julian calendar is a calendar system, while JDN is a day count. The two are related but not the same. The Julian calendar is still used by some Eastern Orthodox churches, but JDN is used in astronomy and computing.

Also, note that the astronomical Julian Day starts at noon, not midnight. This is because the astronomer’s day was traditionally considered to start at noon. If you are working with civil dates, you may need to adjust by 0.5 day.

Limitations and Precision

Julian Day Numbers can be very large (currently around 2.46 million), which can cause floating-point precision issues in some programming languages. For most applications, a double-precision float is sufficient, but for exact calculations, it is better to use integer arithmetic or a specialized library.

The formula works for dates in the Gregorian calendar, which is the standard civil calendar today. For dates before 1582, the Julian calendar was in use, and you may need to specify which calendar system you are using. Many converters allow you to choose between Julian and Gregorian for historical dates.

Frequently Asked Questions

What is the difference between Julian Day Number and Julian Date?

Julian Day Number (JDN) is the integer count of days since the start of the Julian period (noon, January 1, 4713 BCE). Julian Date (JD) is the JDN plus the fractional part of the day, representing the exact time. For example, at 18:00 UTC on June 15, 2025, the JDN is 2460467, and the JD is 2460467.25.

Why does the Julian Day start at noon?

The astronomical convention starts the day at noon because it was established by Joseph Scaliger in 1583, and later adopted by astronomers. The reason is that when observing the sky at night, the date would not change in the middle of an observation session. This convention is still used in astronomy, but many software applications use midnight as the start for civil purposes.

Can I convert a Julian Day Number to a date?

Yes, you can. The reverse formula is more complex, but many online converters and programming libraries can do it. For example, the Julian Day Number 2460467 corresponds to June 15, 2025, at noon UTC. If you have a fractional part, you can convert it to hours, minutes, and seconds.

What is the current Julian Day Number?

As of June 15, 2025, the Julian Day Number at noon UTC is 2460467. The number changes each day at noon. You can find the current JDN from an online converter or by using a formula in a spreadsheet.

Are Julian Day Numbers used in programming?

Yes, JDN is useful in programming for date arithmetic, such as calculating the difference between two dates or sorting dates. Many programming languages have built-in functions to convert dates to Julian Day Numbers. For example, Python’s datetime library can do this, and JavaScript has libraries like julian.

Related

Formula used
in d | Date (YYYY-MM-DD) | - | 2025-06-15 | 0
in t | Time (HH:MM) | - | 12:00 | 0
out jdn | Julian Day Number | days | jdn_from_date(d) + (hour(t) + minute(t) / 60) / 24 - 0.5 | 5

Each line is one input (in, sel, list) or one result (out: label, unit, formula, decimals). How the calculators work

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.