The Gregorian Calendar and Why Old Dates Differ

Two calendars

Most of the world today uses the Gregorian calendar for civil purposes. Before it, much of Europe used the Julian calendar, introduced by Julius Caesar in 45 BC. The two calendars agree on the months and on the number of days in each, and they differ in one thing: which years are leap years.

The Julian calendar has a leap year every four years without exception. The Gregorian calendar omits three leap years in every 400 by skipping the century years that are not divisible by 400. The result is a small difference in the average year length, and a difference in dates that grows over the centuries.

Why the calendar was reformed

The Julian year is about 11 minutes longer than the true solar year. Over many centuries the calendar drifted against the seasons, and by the sixteenth century the spring equinox fell about ten days earlier on the calendar than it had in the fourth century, when the date of Easter was fixed by reference to it.

Pope Gregory XIII ordered a reform in 1582. Ten days were dropped to bring the calendar back in step, and the leap-year rule was changed to the one we use today. In the Catholic countries that adopted it at once, Thursday 4 October 1582 was followed by Friday 15 October 1582.

The Gregorian rule gives an average year of 365.2425 days and keeps the calendar in step with the seasons for thousands of years.

Countries switched at different times

The reform was not adopted everywhere at once. Catholic countries such as Spain, Portugal, Poland-Lithuania and most Italian states changed in 1582, and France followed in December of that year. Many Protestant and Orthodox regions waited for a century or more, because the change came from the Pope.

The Protestant states of Germany and Denmark-Norway changed in 1700. Great Britain and its colonies changed in 1752, when Wednesday 2 September 1752 was followed by Thursday 14 September 1752, a gap of eleven days. Sweden changed in 1753. Japan adopted the Gregorian calendar in 1873. Russia changed in February 1918, when 1 to 13 February were dropped. Greece followed in 1923.

Between 1582 and 1923 a date could therefore mean different days depending on the country. A letter dated 1 March in England could have been written eleven days after one with the same date in France.

Old Style and New Style dates

Historians use the labels Old Style and New Style to say which calendar a date is given in. Old Style means Julian and New Style means Gregorian. George Washington is the standard example: he was born on 11 February 1731 Old Style, which is 22 February 1732 New Style, because eleven days separated the calendars and the English legal year then began on 25 March.

The gap between the calendars grew with the years. It was 10 days in 1582, 11 days in the eighteenth century, 12 days in the nineteenth century, and has been 13 days since 1900. It will be 14 days after the year 2100, which the Julian calendar treats as a leap year and the Gregorian does not.

Some churches still use the Julian calendar for religious dates, which is why Orthodox Christmas falls in January in some places.

Dates before 1582 in a calculator

A modern date calculator has to choose a calendar for old dates. The usual choice, which this calculator makes, is to apply the Gregorian rules to every year, including years before 1582. This is called the proleptic Gregorian calendar. It is also what the ISO 8601 standard and most programming languages use.

The advantage is consistency: every date has one clear meaning, and the number of days between two dates is always correct under one rule. The disadvantage is that old dates do not match the dates that people wrote at the time. A document dated 4 July 1500 in the Julian calendar is a different day in the proleptic Gregorian calendar.

The weekday of a historical date also depends on the calendar. Weekdays have continued without interruption, so the weekday of 4 October 1582 and of 15 October 1582 differ by only one day, even though ten dates were skipped.

What this means for you

For anything in the last few centuries in a country that had already switched, the difference is irrelevant. For family history in Great Britain before 1752, for events in Russia before 1918 and for dates in Orthodox sources, check which calendar the source used.

A date calculator is still useful for old dates, with a caveat. It tells you the number of days between two dates in the Gregorian calendar. If you need the real elapsed time between two events recorded in the Julian calendar, convert one of them first, or use a tool made for historical dates.

For dates after the changeover in your country, and for anything modern, the calculator can be trusted without special care.

Using the calculator

Enter any date from the year 1 to 9999. The calculator applies the Gregorian rules throughout, shows the weekday and the day of the year, and tells you whether the year is a leap year under those rules.

Try it with 29 February 1900, which does not exist, or 1 January 2100, to see the century rule at work. The calculator treats 1900 and 2100 as ordinary years, and 2000 and 2400 as leap years.