Use the free tool below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.
Two calendars, one confusing history
For most of recorded history, Europe used the Julian calendar, introduced by Julius Caesar. It gave every fourth year an extra day, which made the average year 365.25 days long. That is slightly longer than the true solar year, so over centuries the calendar drifted against the seasons by about one day every 128 years. By the sixteenth century the drift had reached ten days, and Pope Gregory XIII reformed the calendar in 1582, dropping ten days and changing the leap-year rule so century years are leap years only when divisible by 400. That Gregorian calendar is the one most of the world uses today.
The trouble for researchers is that countries did not all switch at once. Catholic countries adopted the Gregorian calendar in 1582, but Britain and its colonies waited until 1752, Russia until 1918, and Greece until 1923. A date written in London in 1700 and a date written in Paris in 1700 therefore used different calendars, and the same physical day had different dates in the two cities. Worse, some records are marked “old style” or “new style”, some are double-dated across the new year, and many are not marked at all. Converting a date without knowing which calendar it used, and which country wrote it, is guesswork.
How to use the converter
- Choose the direction: Julian to Gregorian, or Gregorian to Julian.
- Enter the year, month and day as written in the record.
- Read the converted date.
- Check which calendar your record’s country used in that year before you trust the conversion.
The size of the gap between the calendars grows over time, because the Julian calendar keeps leap days that the Gregorian calendar drops in century years. From 1582 to 1700 the gap is ten days. After February 1700 it becomes eleven, after February 1800 twelve, and after February 1900 thirteen. The converter handles all of that automatically through proper day-count arithmetic, so you do not need to memorise the table. You do need to remember that the gap depends on the date itself, not just the year, because a Julian date in January 1700 still sits in the ten-day period, while a Julian date in March 1700 sits in the eleven-day period.
A worked example
The most famous conversion is the start of the Gregorian calendar itself: Thursday 4 October 1582 in the Julian calendar was followed by Friday 15 October 1582 in the Gregorian calendar, a jump of ten days. Enter 1582, 10, 4 as a Julian date and the converter returns 1582, 10, 14, which is the same physical day in Gregorian terms, not the following day. The distinction matters: the converter translates one date into the other calendar’s name for the same day; it does not move you forward or backward in time.
A second example shows the growing gap. A Julian date of 1 January 2000 converts to 14 January 2000 Gregorian, a thirteen-day gap. That is why Orthodox Christmas, celebrated on 25 December in the Julian calendar, falls on 7 January in the Gregorian calendar: 25 December plus thirteen days is 7 January. If you meet a January date in a Russian record from before 1918, assume it is Julian unless the record says otherwise, and convert it before comparing it with events dated in Western Europe.
Traps in old dates
- Different countries switched in different years: never assume one universal switch date.
- The new year did not always start on 1 January: England started it on 25 March until 1752.
- Double dates such as “12 February 1731/32” mark the January-to-March new-year gap.
- Quaker records used numbers for months, because they refused pagan month names.
- Some “old style” labels mean Julian, but in some contexts they mean the old new-year start.
Research aid only, with the cutover caveat stated plainly: there is no single universal switch date, and different countries switched in different years, so a conversion is only as good as your knowledge of which calendar the record’s writer used. Treat converted dates as working dates for comparison, and confirm them against primary sources and the calendar history of the record’s country before you publish them as fact.