BYTETOOLS

Julian Date Converter

Convert between Gregorian dates, Julian Day Number, Modified Julian Date and the 5-digit YYDDD Julian code used on manufacturing date stamps.

Dates use the proleptic Gregorian calendar throughout, so dates before the October 1582 reform are extrapolated backwards rather than converted from the historical Julian calendar.

What is the Julian Date Converter?

The ByteTools Julian Date Converter handles both things people mean by Julian date. The astronomical one is the Julian Day Number — a continuous count of days since 1 January 4713 BC — together with its Modified Julian Date offset.

  • Julian Day Number, fractional Julian Date and Modified Julian Date
  • 5-digit YYDDD manufacturing date codes, both directions
  • Fliegel–Van Flandern algorithm with exact integer arithmetic
  • Time-of-day input for the fractional part of the Julian Date
  • Handles dates far into the past and the future
  • Copyable results, all computed offline

How to use the Julian Date Converter

  1. 1

    Pick the direction: from a calendar date, or from a Julian value.

  2. 2

    Enter the date and optionally a UTC time of day for the fractional part.

  3. 3

    Read the Julian Day Number, fractional Julian Date, MJD and YYDDD code.

  4. 4

    To go the other way, paste a JDN, MJD or YYDDD code and read the calendar date.

  5. 5

    Copy whichever value you need.

About the Julian Date Converter

The ByteTools Julian Date Converter handles both things people mean by Julian date. The astronomical one is the Julian Day Number — a continuous count of days since 1 January 4713 BC — together with its Modified Julian Date offset. The industrial one is the 5-digit YYDDD code stamped on food, pharmaceutical and aerospace packaging.

Conversion runs in both directions. Enter a calendar date and get the Julian Day Number, the fractional Julian Date for a given time of day, the Modified Julian Date and the YYDDD code. Or enter any of those and get the calendar date back, using the standard Fliegel–Van Flandern algorithm in both directions.

It is used by astronomers reducing observations, satellite and GPS operators, and warehouse staff decoding lot codes. Everything is integer arithmetic that runs in your browser — no lookup service and no upload.

Frequently asked questions

What is a Julian Day Number?

It is a continuous count of days starting from noon on 1 January 4713 BC in the proleptic Julian calendar. Because it is a single running number with no months or leap years, astronomers use it to compute intervals between observations without any calendar arithmetic.

What is the difference between JD and MJD?

Modified Julian Date is simply the Julian Date minus 2400000.5. That subtraction gives a shorter number and shifts the day boundary from noon to midnight, which is more convenient for satellite tracking and everyday scientific logging.

How do I read a 5-digit Julian date code on a product?

The first two digits are the year and the last three are the day of the year, so 25046 means the 46th day of 2025, which is 15 February. Some manufacturers use a 4-digit YDDD form instead, where a single digit gives the year within the current decade.

Why does the Julian Day start at noon?

The convention comes from astronomy, where a single night's observations should share one date rather than being split across midnight. Starting the count at noon keeps an entire observing session on the same Julian Day.

Does this converter work for dates before 1582?

The tool uses the proleptic Gregorian calendar throughout, so dates before the October 1582 reform are extrapolated backwards rather than converted from the historical Julian calendar. For historical research before that date, check which calendar your source is using.

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