753 BC – Roman calendar
Calculated from the founding of the city of Rome, generally agreed to be 21 April, 753 BC, the Roman year had 304 days and was divided into ten months, with 61 days of winter not assigned to any month. The years were not counted; instead they were named after the consuls who were in power at the start of the year. Around 703 BC, Numa Pompilius, the second King of Rome, added January and February to make a twelve-month or 355-day year.
45 BC – Julian calendar
Named after Julius Caesar, the Julian calendar was introduced to approximate a tropical year (the length of time that the Sun takes to return to the same position in the cycle of seasons.) Comprising 365 days divided into 12 months, there was a leap day added to February every four years, meaning each year was on average 365.25 days long.
1582 – Gregorian calendar
Pope Gregory XIII introduced the Gregorian calendar after establishing that the Julian calendar was about ten days out of date. Because the tropical year is actually 11 minutes shorter than the 365.25 days the Julian calendar dictated, it meant a gain of about three days every four centuries. The equinox was occurring on 11 March and since it is a date so tied to Easter, the Roman Catholic Church instigated the change. Three leap year days were dropped across every four centuries to combat the problem.
1752 – England adopts the Gregorian calendar
Until 1752, England still used the Julian calendar but mistakes were frequently made because much of Europe had already adopted the Gregorian calendar. The Calendar Act of 1750 reformed the English calendar so that a new year begun on 1 January, rather than 25 March. This resulted in 1751 only having 282 days, running from 25 March to 31 December. 1752 began on 1 January.
2000 – The Y2K problem
Since the advent of computers, it was common practice to use two digits rather than four to represent a year in systems, as a way to save memory. As the 1990s approached, experts began to realise that in 2000, computer systems would interpret 00 as 1900, causing havoc. Companies and organisations worldwide checked, fixed, and upgraded their computer systems accordingly. The cost of the work is estimated to have been more than $300m.