By the year 50 BC, Rome’s year was quite out of step with the seasons: the 1st January was occurring in the autumn. Julius Caesar, among his reforms, introduced a new calendar based on the latest research by Egyptian astronomers, who had now calculated the length of the year to be 365.25 days. In his capacity of Pontifex Maximus, Caesar intercalated enough days to bring the civic year into coincidence with the solar year. From that time onwards, Europe used Caesar’s new calendar—the Julian Calendar, with the number of days in each month that are still used, including the extra day every four years.
However the calculation of 365.25 was not quite accurate. The tropical year, that is the passage of the sun from solstice to the same solstice, is nearer to 365.2422 days.
This error amounted to little over one day in 130 years, but it was enough to be noticed as early as AD 730, when the Englishman St Bede noticed in his monastery at Jarrow that the vernal equinox was occurring several days earlier than its position on the calendar.
However, it was not till 1582, when once again the calendar was obviously out of step with the seasons, that Pope Gregory XIII reformed the calendar. The new version began to be called the Gregorian calendar—which is slightly misleading, because it involved merely adjustments.
But Gregory’s reform was not adopted by Protestant England until 1752. In that year the 2nd September was followed by the 14th, to catch up with the tropical year.
People had to decide what to do about birthdays and other anniversaries. If they chose to keep the same date, they were required to use the letters OS after it, meaning Old Style. But most people preferred to move their birthday to fit the new calendar.
In the literature of that time, students may notice writers referring to the date of a recent event with OS after it; and this was necessary in legal documents.
The rest of the Gregorian changes were also adopted—involving the omission of the leap year at the beginning of every century unless that year figure is divisible by 400. It was also at this time that New Year’s Day was moved from the 25th March to the 1st January.
Minor irregularities that remain are now corrected electronically, and most countries in the world have access to the time mechanism that is broadcast by radio and by the internet.
The Eastern Orthodox churches still use the Julian calendar to date their ecclesiastical year, which is why, for example, they celebrate Christmas on the 7th January.
Note for students: Easter, in both Eastern and Western churches, is dated in the same way as the Jewish Passover: it follows the first full moon after the vernal equinox.