What is a Julian Date (JD)?

A Julian Date is a continuous count of days and fractional days since noon UTC on 1 January 4713 BC (in the proleptic Julian calendar). Unlike calendar dates, a JD is a single decimal number, so the time between any two events is just a subtraction. For example, JD 2451545.0 is exactly 2000-01-01 at 12:00 UTC — the standard J2000.0 epoch used in orbital mechanics.

What is the difference between JD, JDN, and MJD?

The Julian Day Number (JDN) is the integer part: it increments at noon UTC and counts whole days. The Julian Date (JD) adds the fractional part — a value between 0 and 1 representing the fraction of a day past the preceding noon. The Modified Julian Date (MJD) is JD minus 2,400,000.5, which shifts the day boundary from noon to midnight and gives more compact numbers for modern dates (e.g. MJD 51544.5 = J2000.0).

How is the Julian Date calculated from a Gregorian date?

The JDN is found with the Richards / Fliegel-Van Flandern integer formula: given year Y, month M, day D, compute a = floor((14 - M) / 12), then JDN = D + floor((153*(M + 12*a - 3) + 2) / 5) + 365*(Y + 4800 - a) + floor((Y+4800-a)/4) - floor((Y+4800-a)/100) + floor((Y+4800-a)/400) - 32045. The fractional day is (hour - 12)/24 + minute/1440 + second/86400, and JD = JDN + fraction.

What is the J2000.0 epoch?

J2000.0 is the standard astronomical reference epoch defined as JD 2451545.0, which corresponds to 2000 January 1.5 TT (Terrestrial Time, approximately 2000-01-01 12:00:00 UTC). Orbital elements, star catalogues, and coordinate systems are often specified at J2000.0. The J2000.0 offset shown by this tool is simply JD minus 2451545.0 in days.

What is the Julian calendar date shown as "O.S."?

O.S. stands for Old Style — the proleptic Julian calendar, which predates the Gregorian reform of 1582. Before October 1582, the Julian and Gregorian calendars agree on day numbering but not on the date: for example, the same moment that is 2000-01-01 in the Gregorian calendar is December 19, 1999 in the Julian calendar (they drift by ~13 days in the modern era). Historical events in Russia, Greece, and other countries that adopted the Gregorian calendar late are often recorded in Old Style dates.

Why does the Julian Date increment at noon instead of midnight?

The convention dates from the early history of astronomy. When the Julian Day system was introduced in 1583 by Joseph Justus Scaliger, astronomical observations were typically made at night and recorded by date. Starting the JD count at noon meant that a single night of observations always fell within a single Julian Day, avoiding the ambiguity of a midnight boundary splitting one observation session across two calendar days.

Julian Date Converter

The Julian Date (JD) is the gold standard for time-stamping events in astronomy, orbital mechanics, and scientific computing. Instead of fragmented year/month/day/hour fields, a JD is a single decimal number that grows monotonically — the elapsed time between two events is just one subtraction away. This converter translates between Gregorian calendar dates (with full UTC time resolution) and the family of Julian Date quantities: JD, JDN, MJD, and J2000.0 offset.

How it works

The tool runs two directions entirely in your browser — no API calls, no rounding by a remote server.

Gregorian to JD uses the Richards integer formula to find the Julian Day Number (JDN), then adds a fractional day measured from noon UTC:

a   = floor((14 - month) / 12)
y'  = year + 4800 - a
m'  = month + 12*a - 3
JDN = day + floor((153*m' + 2) / 5) + 365*y'
          + floor(y'/4) - floor(y'/100) + floor(y'/400) - 32045

JD  = JDN + (hour - 12)/24 + minute/1440 + second/86400

The three derived quantities follow automatically:

MJD = JD - 2,400,000.5 — same count but starting at midnight; used in space geodesy and satellite tracking
J2000.0 offset = JD - 2,451,545.0 — days before or after the standard epoch 2000-01-01 12:00 UTC
Julian-calendar date (O.S.) — the equivalent date on the proleptic Julian calendar, useful for history before 1582

JD to Gregorian inverts the formula using the Richards reverse algorithm, splitting the integer JDN back into year, month, and day, then converting the fractional part into hours, minutes, and seconds.

Worked example

The J2000.0 epoch is one of astronomy’s most-cited reference points. Enter 2000-01-01, 12 h, 0 m, 0 s in Gregorian mode and you get:

Quantity	Value
JD	2451545.00000000
JDN	2451545
MJD	51544.50000000
J2000.0 offset	0.000000 days
Day of week	Saturday
Julian calendar (O.S.)	December 19, 1999

Try 1969-07-20 (Apollo 11 Moon landing day) at 20:17:40 UTC:

JD ≈ 2440423.345602
MJD ≈ 40422.845602
J2000.0 offset ≈ -11122.654 days (about 30.5 years before J2000)

The timeline diagram

Below the results the tool draws a compact SVG timeline from JD 0 (noon UTC, 1 Jan 4713 BC) through the present, with tick marks at the Gregorian reform (1582), the 1900 epoch, J2000.0, and the approximate current date. A blue marker shows where your chosen date falls. This gives an instant sense of scale — J2000.0 is only about 0.8% of the way along the full JD range from 0 to today.

Every calculation runs locally in your browser. No date or time information is transmitted anywhere.