The Julian calendar
Decreed by Julius Caesar in 46 BC and in use from 45 BC, the Julian calendar had 365 days with a leap day every four years without exception, for an average year of 365.25 days. The real solar year is about 365.2422 days, roughly 11 minutes shorter, so the calendar drifted by about one day every 128 years.
The 1582 reform
By the 16th century the drift had reached about ten days. In 1582 Pope Gregory XIII issued the Gregorian calendar, which:
- Skipped ten days: Thursday 4 October 1582 was followed directly by Friday 15 October 1582. The weekday sequence was not interrupted.
- Refined the leap-year rule: century years are leap years only if divisible by 400 (2000 was; 1900 and 2100 are not), giving an average year of 365.2425 days. See Leap years and 29 February birthdays.
Gradual adoption
Some European countries switched in 1582; Britain and its colonies switched in 1752, when the gap had grown to 11 days; others waited until the 20th century. That is why one historical event can have two different dates. The Julian–Gregorian gap is 13 days today and will remain so until 2100.
What it means for calculators
Our tools use the proleptic Gregorian calendar, meaning today’s rules applied to every date, including before 1582. This is the standard in software and international norms. So a pre-1582 date written in the Julian calendar in a historical source may not match the same date in our tools. The Birth Date & Weekday tool correctly shows 15 October 1582 as a Friday, the first day of the new calendar.
Gregorian and Hijri side by side
In Saudi Arabia both calendars are used: Gregorian in much of business, and Umm al-Qura Hijri for civil transactions and religious occasions. Because the Hijri year is about 11 days shorter, there is no fixed offset between them. See Converting Hijri and Gregorian dates by hand.
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