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Julian date converter: three meanings, and how to tell them apart
Paste a number to see every reading that could produce it, or convert a date into all of them at once.
Which Julian date do you mean?
The phrase is used for at least three unrelated things, and a converter that does not ask which one you have is guessing. The astronomical Julian Day Number is a continuous count of days running from 4713 BC, used in every ephemeris and astronomical calculation; today's is a seven-digit number. A Julian calendar date is a date in the calendar the Gregorian reform replaced, which is what old style means on a historical document. And the Julian date printed on food and medicine is a lot code: two or four year digits followed by the day of the year, so 26215 means the 215th day of 2026.
They have almost nothing to do with each other beyond the word. The lot code is not a count of days from anywhere, the day number is not a calendar, and the calendar is not either of them — so a number alone genuinely does not say which you are holding.
The confusion is not academic. If you type a packaging lot code into an astronomical converter it will happily return a date in the fifth millennium BC, and nothing will look wrong. This page shows every reading side by side and marks which ones give a believable date, so the answer is yours to pick rather than the tool's to assume.
How to use it
- Say what you have. A number you need identified, or a date you want converted. The samples cover a genuinely ambiguous number, one that can only be a Modified Julian Date, one that can only be a lot code, a Julian Day Number, and a number no reading can place.
- Read every interpretation, not just the first. Each reading shows the date it would give and whether that date is believable. A number is only identified when the other readings land somewhere absurd — which is luck, not design.
- Set the century for lot codes. A two-digit year has no century and no standard to supply one. The tool states the window it assumed rather than hiding it, and changing it moves the date by a hundred years.
Why astronomical Julian dates end in .5
Because the astronomical day begins at noon. That is not a quirk of any particular tool — it is the definition, and the cleanest proof is the constant that converts between the two counts. The Modified Julian Date is defined as the Julian Date minus 2400000.5, and the reference implementations of the astronomical standards carry that number as a named constant. If the Julian Date also began at midnight, the offset between two midnight-based counts would be a whole number of days. It is not. The half is the shift from noon to midnight.
That is the whole reason MJD exists. Astronomers wanted a shorter number that changed date at midnight like everyone else's, so they subtracted a large constant and a half day at the same time. A civil midnight has a Julian Date ending in .5 and a Modified Julian Date that is a whole number, and this page shows both for any date you give it.
The practical consequence is that a Julian Date and a calendar date do not line up the way you expect. The Julian Day Number for a given calendar day covers noon on that day through to noon the next, so an observation logged at two in the morning belongs to the previous Julian day. Software that converts by rounding rather than by respecting the noon boundary is off by one for half of every day.
What this cannot tell you
It cannot tell you which reading was intended. Every five-digit number is a valid Modified Julian Date, and of the 90,000 of them, 32,872 are also valid lot codes — so for a third of the range, both readings are available and nothing in the number chooses between them. Where you found it is the only evidence: a number on a yoghurt pot is a lot code, the same digits in a telescope log are not. The tool reports the ambiguity rather than resolving it.
Lot codes have no standard at all. The two-digit year form is the common one and its century is always a guess; some manufacturers use four year digits, some put the day first, some add a plant or shift code on the end, and none of that is written down anywhere binding. A code for 2005 is 05123, and if the leading zero is lost in a spreadsheet you are left with something no reading can place — which is one of the sample buttons, because it happens constantly.
The calendar conversion is arithmetic, not history. The reference implementation of the astronomical standards says as much in its own documentation: no account is taken of the dates of adoption of the Gregorian calendar. Britain switched in 1752, Russia in 1918, and Greece in 1923, so a date from 1700 means different days depending on where it was written. This page converts between the two calendars, which is well defined, and does not attempt to guess which one a historical document was using.
Two smaller things. The believability window here is 1900 to 2100, which is a judgement about what a reader is likely to be holding rather than a fact — a genuine astronomical Julian Day Number for an ancient eclipse is a perfectly good number that this page will mark as implausible. And the ISO 8601 ordinal date, written 2026-215 with a four-digit year and a hyphen, is a different notation from the lot code; the ISO parser on this site handles that form.
Why is it free?
Converting between day counts is arithmetic, and it runs in your browser. There is no server in the loop, so there is nothing to bill for and no account to create.
Nothing is uploaded. Reload the page and it has forgotten your number.