Our current calendar system – 12 months, 365 days give or take an occasional leap day, starting on January 1 – has not been around for as long as you may think. For centuries people followed the calendar system enacted by the Romans. The old Julian calendar, established by Julius Ceasar, divided the year into a eleven months either 29 or 31 days long (superstition forbade the use of 30 days even), plus an extra month of 28 days to round out the length. Slightly modified in year 46 A.D., the calendar was remarkably accurate for its became real time when added up over the centuries, as Christians later found out.

On this day, January 1, in 1583 the Gregorian Calendar, introduced Pope Gregory XIII, officially started. The new calendar moved the date ahead 10 days to correct for the inaccuracies of the Julian calendar. Gregory also ordered century years (1600, 1700) to be leap years only when divisible by 400.

This system is still used by the United States today – since the year 2000 was divisible the 400, it was a leap year as scheduled. It is still not 100% accurate, but comes within 23 seconds – good enough to require a leap day only once every 3,200 years. In the last few decades official timekeepers, requiring precise measurements down to fractions of seconds, have moved away the Sun and moon, and have taken to identifying atomic shifts as the basis for a second. Interestingly, they found out. that due to the Earth’s ever-so-slight irregular rotational wobble, it is impossible to predict to precisely to the tenths how long a year will be.