Julian Date (JD) / MJD Converter

Convert between calendar date/time and astronomical day counts. Get Julian Date (JD), Modified Julian Date (MJD), ISO 8601 (UTC), Unix timestamp, and days since J2000.

Input

Conversion is computed in UTC (we use your local time and convert to UTC internally).

Output

Julian Date (JD)
—
Modified Julian Date (MJD)
—
ISO 8601 (UTC)
—
Unix ms (UTC)
—
Days since J2000.0
—
Notes: JD starts at noon (12:00) UTC by convention. This tool uses UTC and the proleptic Gregorian calendar; it does not apply TT/ΔT.
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What is Julian Date (JD)?

Julian Date (JD) is a continuous count of days (and fractions of a day) since noon UTC on January 1, 4713 BC (Julian calendar). Astronomers, satellite operators and data engineers use JD to avoid ambiguity introduced by calendars, time zones and daylight saving changes.

What is Modified Julian Date (MJD)?

MJD is defined as MJD = JD − 2 400 000.5. It starts at midnight (00:00) UTC and keeps numbers shorter for modern epochs (e.g., MJD ≈ 60 000).

How to convert Date ↔ JD/MJD

We convert using UTC. The core relations are:

  • MJD = JD − 2400000.5
  • JD = MJD + 2400000.5
  • Days since J2000.0 = JD − 2451545.0

Formula (Gregorian, proleptic) for a UTC date/time (Y, M, D with fractional day):

if M ≤ 2: Y' = Y − 1, M' = M + 12 else: Y' = Y, M' = M
A = ⌊Y'/100⌋
B = 2 − A + ⌊A/4⌋
JD = ⌊365.25·(Y' + 4716)⌋ + ⌊30.6001·(M' + 1)⌋ + D + B − 1524.5

Our converter applies this algorithm and its inverse (Fliegel–Van Flandern style) to recover calendar date from JD.

Accuracy, UTC vs TT and leap seconds

For everyday and most engineering uses, JD on UTC is enough. High-precision astrometry often uses Terrestrial Time (TT) (≈ UTC + 69 s around 2025; changes with leap seconds and ΔT). This page purposely sticks to UTC for predictable results and easy cross-checking.

Worked examples

  • 2000-01-01 12:00:00 UTC → JD = 2451545.0 (J2000.0 epoch), MJD = 51544.5
  • 2000-01-01 00:00:00 UTC → JD = 2451544.5, MJD = 51544.0
  • 2024-01-01 00:00:00 UTC → JD = 2460310.5, MJD = 60310.0
  • 1970-01-01 00:00:00 UTC → JD = 2440587.5, MJD = 40587.0

Common pitfalls

  • Noon vs midnight: JD starts at noon UTC; MJD at midnight. Mixing both shifts by 0.5.
  • Time zones: Always convert local time → UTC before calculating JD/MJD.
  • Rounding: Keep sufficient precision (≥ 6 decimals) for sub-minute accuracy.
  • Calendar cutover: We use the proleptic Gregorian calendar for all dates; historical sources may differ before 1582.
  • Leap seconds: JD(UTC) does not “absorb” leap seconds; TT/TAI tracks them differently.

Typical use cases

  • Astronomy logs, ephemerides, photometry series
  • Satellite telemetry and GNSS timestamps
  • Scientific pipelines that need monotonic ordering across time zones
  • Data exchange between systems with different local calendars

Need other time conversions? Try our fast utilities:

FAQ

Why does JD start at noon? Historically, starting at noon avoided date changes during night observations; the convention persists.

What precision do I need? Six decimals in JD (~0.0864 s) is plenty for most uses; more decimals capture subseconds.

What is J2000.0? The standard epoch with JD 2451545.0 (2000-01-01 12:00:00 UTC). We show your offset in days relative to it.

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