system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's the big difference between the Julian and the Gregorian calendar that made the world switch?
gpt
So, astronomers found that a year was approximately 365.25 days, so to account for this, they added a leap year, a year where an extra day is added every four years. So, to the normal 365 calendar, they added this rule.

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- If year is divisible by 4, it is a leap year and an extra day is added at the end of February. 

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 This is known as the Julian Calendar, named after Julius Caesar. However, they were slightly off, which was found by later  astronomers, and they found that a year is slightly less than they thought, so they were adding too many leap years. So a new calendar was made, with these two new rules.

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- A year divisible by 100 is not a leap year. In the Julian Calendar, it was.

- If year is divisible by 400, it is an exception to the rule above.

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This is known as the Gregorian Calendar, named after Pope Gregory XIII, who enforced the conversion to the new calendar. Currently, they are 13 days off, with the Julian calendar ahead by 13 days.

Modern astronomers now know with more precision the length of a year, but Gregorian measurements were precise enough that we won't have to worry about inaccuracies for another tens of thousands of years or so.

Edit: Formatting and elaboration.