Tomorrow, February 29th, is a “leap day”, so remember to set your clocks back by 24 hours! Fortunately, we don’t really have to do that. Even clocks that keep track of the date as well as the hour pretty much all know when we have a leap year.
But do you know WHY we have leap days? Or exactly when? Time is a surprisingly complicated topic, even when you ignore general relativity. Hours, minutes, and seconds are essentially arbitrary units, so we can define them to fit our world however we want. Days and years, on the other hand, are defined by the time Earth takes to complete one revolution (a day) and one orbit around the Sun (a year). And although these two natural (and more or less constant) periods both involve Earth, they are quite independent of each other. It happens that a full year is really about 365.25 days, so every 4 years we “add a day” to the month of February to get our calendars back to proper alignment with Earth’s position in its orbit around the Sun.
But since it’s not exactly 365.25 days (a bit less), we also have to skip the leap day sometimes. The rule goes like this: If the year is evenly divisible by 4 it’s a leap year, except if it’s also divisible by 100, in which case it’s not a leap year, except when it’s also divisible by 400! So 2000 was a leap year (divisible by 400), but 2100 will not be.
As I suggested earlier, many scientific uses of time get much more complicated than this, and some really important pieces of modern technology must even account for the effects of general relativity. For example, GPS systems could not be accurate without it.
Even understanding leap years can be a challenge for technology: It was discovered in the early days of 2016 that the firmware in my Astro-Physics telescope mount has a bug caused by one part of it thinking this is not a leap year, while another part has it right. Depending on how you operate the mount, this can cause the telescope to move in the wrong direction and crash into the mount! The bug has been fixed, but I’m sure it was a painful experience for some users and a major embarrassment for the Astro-Physics company.
For more details on how leap years work: https://en.wikipedia.org/wiki/February_29