We all know that there are 24 hours in a day, 365 days in a year, except for leap years, when there's an extra day added onto the end of February.
But why do we even have them?
A full spin around the sun is actually a quarter of a day longer than we account for, and inserting an extra day into the mix every 4 years keeps things on track.
This may seem a bit confusing and li more trouble than it's worth, but needing to keep track of the days of the year accurately played a very large role in something that arguably had a greater impact on human evolution than anything else.
SciShow host Hank Green explains why keeping a calendar is more convoluted than we thought, and some ways people have tried to keep track of time in the past were incredibly confusing and even less reliable: