Convert dates between Julian and Gregorian calendars with the Julian Day Number and weekday. Free, no sign-up.
What this tool does
Old documents carry Julian dates, and the gap to Gregorian dates grows over centuries: 10 days in 1582, 13 days today. A letter dated 4 October 1582 Julian is 14 October Gregorian — and in Catholic countries the next day after 4 October 1582 was 15 October, because ten days were skipped outright. This converter translates dates both ways, shows the Julian Day Number astronomers use as a neutral count, and gives the weekday, so historical citations stop being off by a fortnight.
Why it matters
Good citations do quiet work. They let a reader find every source you used, they show that your argument rests on real evidence rather than memory, and they protect you: a clean reference list is much harder to challenge than a vague one. Most marks lost on referencing are not lost on difficult sources. They are lost on small, repeated slips — a missing year, a swapped initial, a title in the wrong case — that a quick tool pass catches before submission.
How to use it
- Pick the direction: Julian to Gregorian or back.
- Enter year, month and day.
- Read the converted date, JDN and weekday.
Worked example
Julian 4 October 1582 converts to Gregorian 14 October 1582, a Thursday, JDN 2299160. Gregorian 25 October 2026 converts to Julian 12 October 2026, a Sunday. The tool shows both the date and the weekday so you can cross-check against the document.
Tips for good results
When a document gives both styles (Old Style / New Style), cite both. Readers in different traditions will each find the date they expect.
Work from the source itself, not from another paper’s copy of it. And keep one style for the whole list: a single mixed entry undermines an otherwise perfect bibliography.
Honest limits
Conversion uses the standard astronomical algorithms for the proleptic calendars. Real history is messier: different countries adopted the Gregorian calendar in different years (Britain in 1752, Russia in 1918), so a ‘Julian’ date’s meaning depends on where and when it was written. Always note the country alongside the date.
Common mistakes to avoid
Common mistakes to avoid: copying a reference from another paper without checking it against the original, trusting a PDF filename for the title or year, and mixing two styles in one list. Each of these looks harmless alone and careless together. Run the tool, read its output once, and compare it with the source itself before you paste it into your document.
Before you submit
Run through this short check before you submit: is every author name spelled as it appears on the source, is every year present and correct, and does every in-text citation have a matching entry in your reference list? Read the list once from top to bottom rather than skimming it. Most referencing errors are found in that single slow read, and fixing them there is far cheaper than fixing them after a reviewer finds them. If one entry still looks odd after that read, check it against the original source before you change anything else.
Frequently asked questions
Is this tool really free?
Yes. It runs in your browser on this page, with no account and no payment. Nothing you paste leaves your computer.
Do I still need to check the result?
Yes, once. Tools apply rules; they cannot verify facts about your source. Compare the output with the source itself before submitting.
Can I use the output in my paper?
Yes. The output is a formatted reference or analysis for your own work. Keep the source’s facts accurate and the formatting consistent across your list.
What if the tool’s output differs from my style guide?
Trust the style guide. Tools encode the common rules of a style; your university or journal’s own sheet wins every disagreement. Use the tool to do the routine work, then apply the local rule where it differs.
Also available in Spanish: Conversor de Calendarios Históricos – Conversor de Fechas Juliano a Gregoriano Gratis.