Years which include a leap day, like 2024, are called leap years. However, it’s not as simple as having a leap year every four years exactly.
That’s because the adjustment of an extra day every four years works out at the rate of six hours added per year, which is just a little lpnger than the 5 hours and 48 minutes which are required. So occasionally, leap years are skipped to allow for this.
Generally every 100 years a leap year is skipped. Typically the leap year which is skipped is when a year number is divisible by 100 but not 400. So 2400 will be a leap year, but the years 2100, 2200 and 2300 will not.
This needs to be coordinated carefully across international organizations to ensure everyone is operating on the same time. As well as leap years, sometimes smaller adjustments called leap seconds need to be made to allow for Earth’s rotation.
“Our calendar is constantly in need of monitoring,” Malhotra explained. “Every now and then you’ll hear about a leap second being added to a year. That happens sometimes because the Earth’s rotation speed fluctuates, for example, from continental movements, and it is also slowing down gradually because of lunar and solar tides and friction within our planet. These can affect spin rate and make the planet rotate a little more or a little less in a given year.”
