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Julian Date Converter

Convert between Gregorian calendar dates and Julian Day Numbers (JD) instantly. Supports full date and time in UTC.

Updated

What is the Julian Date Converter?

Convert any Gregorian calendar date (with optional time) to its corresponding Julian Day Number (JD) and back.

This tool provides accurate, instant two-way conversions between civil dates and the continuous day count used in astronomy, space science, and historical calculations. It also shows the Modified Julian Date (MJD) for convenience.

Supports full date + time inputs in UTC and works across a wide historical and future range.

How it works

The Julian Day Number (JDN) counts the number of days since noon Universal Time on January 1, 4713 BCE in the proleptic Julian calendar. The fractional part represents the time of day since noon (0.0 = noon UT).

Conversion Formulas

This tool uses standard conversion formulas with the proleptic Gregorian calendar (the Gregorian rules are extended backward before their historical adoption):

  • Date → JD: Adjusts month/year for January/February, applies the standard integer formula with time fraction added.
  • JD → Date: Uses the inverse algorithm to extract year, month, day, and time components.

All calculations are performed client-side in UTC. The tool also derives the Modified Julian Date (MJD = JD - 2,400,000.5), which starts at midnight and is commonly used in modern astronomy.

Accuracy & Range

  • Supports dates from roughly the Julian Day epoch onward, using proleptic Gregorian rules throughout.
  • Handles leap years consistently; dates before the Gregorian reform are not converted using the historical Julian calendar.
  • Precision to sub-second level for time components.

Examples

Convert modern date to JD

Find the Julian Day Number for a specific date and time.

2024-07-10 12:00:00
 
2460502.0

Convert JD to Gregorian date

Reverse lookup from Julian Day back to calendar date.

2460000.5
 
2023-02-25 00:00:00

Today's Julian Date

Quick reference for current UTC Julian Day.

today
 
Current UTC date and time converted to Julian Day and Modified Julian Day.

Frequently asked questions

What is a Julian Day Number (JD)?

The Julian Day Number is a continuous count of days since noon UT on January 1, 4713 BCE. It is widely used in astronomy and scientific computing because it avoids calendar irregularities and leap year complications.

What is the difference between JD and MJD?

Modified Julian Date (MJD) = JD - 2,400,000.5. It starts at midnight instead of noon and drops the leading "24" digits for simplicity in the modern era.

Does this tool support time of day?

Yes. Enter dates with time like "2024-07-10 14:30:00" for precise fractional JD. The output includes hour/minute/second breakdown.

Which calendar system is used?

The tool uses the proleptic Gregorian calendar (extended backward). Dates before the historical Gregorian reform (1582) are handled consistently with astronomical standards.

Can I convert dates before 1582?

Yes, the algorithms support ancient dates (back to ~4713 BCE). Results follow proleptic Gregorian rules.

Why does JD start at noon?

Historically, astronomical days began at noon to keep observations within a single day number. The fractional part represents hours since noon UT (0.0 = noon, 0.5 = midnight).

Is this accurate for astronomical use?

It converts between calendar dates and Julian Day values; it does not calculate astronomical positions or account for differences between time scales such as UT and TT. For observational or high-precision work, check the time-scale requirements and verify results with an appropriate astronomy reference.

How do I use "today" or current time?

Click the "Use Today" button or enter a recognizable date string. The tool automatically uses UTC.

What formats does the input accept?

For Date → JD: YYYY-MM-DD or YYYY-MM-DD HH:MM:SS. For JD → Date: any decimal number like 2460523.5.

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