Why historical sources give two different dates for the same event: Julian vs Gregorian calendars

The problem

Two reputable sources can disagree about the date of the same event without either being wrong. A frequent cause is that they use different calendars. The Gregorian calendar was introduced in 1582, but each state decided for itself when to switch from the Julian calendar. Until it did, its records kept using Julian ("Old Style") dates, while countries that had switched used Gregorian ("New Style") dates.

A standard example

Isaac Newton was born in England, which still used the Julian calendar. The Newton Project gives his birth as Christmas Day 1642 by the Julian calendar, which is 4 January 1643 by the Gregorian. At that time Gregorian dates ran ten days ahead of Julian ones. A biography using Old Style dates and one converting to New Style will therefore disagree on both the day and the year.

Why the gap changes

The Julian calendar has a leap year every four years; the Gregorian skips leap years in century years not divisible by 400. Each such skipped year widens the gap by one day:

• 10 days from 1582 until February 1700 (Julian)
• 11 days in the 1700s
• 12 days in the 1800s
• 13 days from March 1900 until 2100

Britain and its colonies switched in 1752, dropping 11 days. Other states switched much later; Wikipedia lists Russia in 1918 and Greece in 1923 for civil use.

Practical guidance

• Check which calendar a source uses; scholarly works often mark dates "O.S." or "N.S.".
• When comparing events across countries before the 20th century, convert to one calendar first.
• Record the calendar alongside any historical date you store.

Limits and caveats

Calendar differences are only one cause of date conflicts; transcription errors, different events being dated (baptism vs birth), and differing year-start conventions also occur. The Gregorian dates for events before 1582 are proleptic, i.e. computed retroactively, and were never used at the time.