This Saturday, you have the gift of time. Feb. 29 is a leap day — a calendar oddity that gives us an extra day.

You probably know why: The time it takes Earth to rotate on its axis is called a day — but it doesn’t take an even number of days to complete a single loop around the sun, or one orbit. Instead it takes a messy 365.2422 spins. And yet the calendar year runs out after 365 days. That means that when the clock strikes midnight on New Year’s Eve, Earth hasn’t quite circled all the way back to its starting point.

“It’s like being a quarter of a day behind at the end of every workday,” said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology. “After four days, you would need one full day to catch up on all your work. It’s the same for the Earth’s orbit and the calendar.”

So every four years, the month of February has 29 days instead of 28. But even that solution isn’t perfect, because the year is not exactly 365.25 days. We have to make additional tweaks. If a year is divisible by 100, for example, there’s no extra day — unless the year is divisible by 400. In other words, the year 2000 was a leap year, but 1900 was not, nor will the year 2100 be one (its nearest leap years will be 2096 and 2104).