How accurate is the Gregorian calendar?
In 3236 of years the calendar shifts 1 extra day in error over to the astronomical one.
Following the Gregorian reform, the average calendar year length is now 365,2425 days, an even better approximation to the astronomical year (≈ 365.242216).
At this rate, it takes about 3236 years for the Gregorian calendar to get into errors, gaining 1 extra day versus the astronomical year, from the moment of its creation in 1582.
By then though we'll probably come up with another better calendar, in advance real time computer synchronized with the astronomical year, based on real time measurements.
Every calendar is just an approximation.
To make a calendar a better measurement of the astronomical year, determined by the Earth's orbit around the Sun, leap year rules were created and have since been improved. These improvements have resulted in a better synchronization between the calendar year and the astronomical one, but the calendar year will only be an approximation:
- The approximate error in the Gregorian calendar: 1 day every 3236 years.
- The approximate error in the previous Julian calendar: 1 day every 128 years.