Julian Date Converter - Gregorian to Julian Day Number

How to use this tool

1. Choose whether to convert a Gregorian date or a Julian Day Number.
2. For a Gregorian date, enter the year, month, day, and UTC time; invalid calendar dates are rejected.
3. Read the JDN, selected-time JD and MJD, midnight and noon reference values, and calendar details.
4. For a reverse conversion, enter a whole-number JDN to see its Gregorian and Julian calendar dates.

Astronomical applications

Julian Dates are a continuous time coordinate used throughout astronomy. They make it easier to compare observations across months, leap years, and calendar reforms. For example, astronomers can subtract the Julian Dates of two observations to find the elapsed time between them.

• Calculating the period of variable stars or binary orbits
• Ephemeris computation (predicting positions of planets, comets, and spacecraft)
• Converting between terrestrial, barycentric, and proper time scales
• Archiving and comparing observations taken decades apart

Epoch reference

What is a Julian Day Number?

The Julian Day Number (JDN) is a continuous whole-number count of days since the beginning of the Julian Period on January 1, 4713 BC (proleptic Julian calendar). This converter uses the proleptic Gregorian calendar for every input date, including dates before the Gregorian reform; it does not automatically switch to the historical Julian calendar.

Julian Date vs Julian Day Number

The Julian Date (JD) extends the JDN to fractional days. By convention, each integer JD falls at noon UTC, so a date’s midnight value ends in .5 and its noon value is an integer. The time fields add the fraction of a day containing 86,400 seconds after midnight. This simple conversion does not model leap seconds or convert UTC to TT, TAI, or UT1; those scales matter for high-precision scientific work.

Modified Julian Date (MJD)

The Modified Julian Date was introduced by the Smithsonian Astrophysical Observatory in 1957 to reduce the large numbers involved. MJD = JD − 2400000.5. The MJD epoch (MJD 0.0) is midnight at the start of November 17, 1858.

Julian calendar vs Gregorian calendar

The Julian calendar (introduced by Julius Caesar in 46 BC) adds a leap year every four years without exception. The Gregorian calendar (1582 reform) skips century leap years unless divisible by 400, making it more accurate. In 2026, the Julian calendar is 13 days behind the Gregorian calendar; after February 28, 2100, the difference becomes 14 days.

Julian dates in other fields

“Julian date” can also refer to an ordinal date used in manufacturing and logistics, often written as a two-digit year followed by a three-digit day of year (YYDDD). That format is not the astronomical Julian Date or the JDN used by this converter. For example, the astronomical JDN identifies a day on a continuous timeline, while YYDDD identifies a day within a particular year.

The Gregorian-to-JDN calculation applies integer calendar arithmetic, then adds the entered UTC time as a fraction of a day: midnight begins at JDN − 0.5 and noon is JDN. This is useful when comparing timestamps for observations, historical events, or dated datasets. Always confirm the calendar convention and time scale expected by the other source before comparing values.