ExoAtlas Tools

Epoch Calculator

Convert between Gregorian date, Julian Date (JD), Modified Julian Date (MJD), and J2000 epoch days. Current values update live in UTC.

Current Epoch Values

Live · UTC

Gregorian Date

Julian Date (JD)

Modified Julian Date (MJD)

J2000 Epoch Days

Convert Epoch Date Format

Enter a value in any one field, then select Calculate.

Results

Gregorian Date & Time —

Julian Date —

Modified Julian Date —

J2000 Epoch Days —

Reference Notes

An epoch in astronomy is the reference time used for consistency in calculation of positions and orbits. Common astronomical epochs include Julian Date, Modified Julian Date, and J2000.

Gregorian Date

The calendar system most used today, introduced in 1582 to correct inaccuracies in the Julian calendar.

Aligned with the solar year to ensure seasonal consistency.
Uses a year, month, and day format.
A year is 365 days, with a leap year every 4 years.
Months have varying lengths (28–31 days).

Julian Date (JD)

A continuous count of days that simplifies astronomical calculations by eliminating months and years.

Starts from 12:00 UTC on January 1, 4713 BCE.
Expressed as a decimal to represent fractional days.
2451545.0 corresponds to January 1, 2000 at 12:00 UTC.

Modified Julian Date (MJD)

A simplified Julian Date introduced for convenience in scientific and technical fields.

Starts from 00:00 UTC on November 17, 1858.
MJD = JD − 2400000.5

J2000 Epoch

A standard reference epoch used for celestial coordinates and astronomical phenomena.

Starts from 12:00 UTC on January 1, 2000.
Expressed in Julian Date as 2451545.0.

Frequently Asked Questions

What is the J2000 epoch?

The J2000 epoch is noon (12:00 TT) on 1 January 2000, equal to Julian Date 2451545.0. It is the standard reference epoch used for celestial coordinates and orbital elements.

What is Julian Date 2451545.0?

Julian Date 2451545.0 is the Julian Date of the J2000.0 epoch, which is noon (12:00 TT) on 1 January 2000.

How do I convert Julian Date to Modified Julian Date?

To convert Julian Date to Modified Julian Date, subtract 2400000.5 from the Julian Date: MJD = JD - 2400000.5.

How do I convert a J2000 epoch day count to a calendar date?

Add 2451545.0 to the J2000 day count to get the Julian Date (JD = J2000 days + 2451545.0), then convert that Julian Date to a Gregorian calendar date. Enter the J2000 day count in the calculator above to do this automatically.

What is the difference between JD, MJD, and J2000 days?

Julian Date (JD) is a continuous count of days from noon on 1 January 4713 BCE. Modified Julian Date (MJD) equals JD minus 2400000.5 and starts at midnight on 17 November 1858. J2000 days count from the J2000 epoch at Julian Date 2451545.0 (noon on 1 January 2000). All three measure the same time on scales that differ only by fixed constants.

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