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March 19, 2024

How to Determine Leap Years on a Linux System


Leap years are not quite as consistent as you may think. A handy Linux script can quickly inform you if a year is going to be a leap year. Although it seems straightforward that every fourth February would have 29, not 28 days, the reality is somewhat more complex. In fact, a leap year is every fourth year (years divisible by 4 with no remainder), just like 2024. However, there’s a catch, years divisible evenly by 100 don’t count as leap years, unless they’re also evenly divisible by 400. This might sound novel, but this rule has been around since 1582, thanks to the Gregorian calendar.

Here’s an example: if you’re still around in February 2100, you might observe that it only has 28 days. February 2400, just like February 2000, will have 29 days. Given the rarity of 28-day Februaries in years divisible by 4, it is not surprising for most people to overlook that they sometimes only have 28 days. Although February 2000 wasn’t too long ago, it kept its extra 29th day, as it was divisible by both 100 and 400. February 2100 won’t be so lucky.

The seemingly peculiar method of determining which February gets an additional day and which doesn’t is logically sound and clever. It’s common to think that the Earth takes 365 days to orbit the Sun, but in reality, it takes approximately 365 days, 5 hours, 48 minutes, and 56 seconds. The sporadic, not quite every four years, addition of an extra day helps keep our seasons relatively consistent.

There’s no direct impact on Linux, yet it’s possible to craft an easy script that determines whether a year is a leap year (featuring a 29-day February). I’ve whipped up an example script for you.

The initial set of instructions merely ascertains whether an argument has been supplied. If absent, it requests one. The next section ensures received arguments are numeric by matching them with a number-representing regular expression.

Should the inputted year be divisible by 400 without remainder, the script declares it a leap year, then reaffirms this conclusion by presenting a calendar for February of that year. If not, it then examines whether it divides by 100 flawlessly. If it does, the year isn’t a leap year. It then examines whether the year divides evenly by 4. Since it has already excluded years that divide by 100 without remainder, but not 400, any affirmative result determines the leap year status. If not, it once again affirms that it is NOT a leap year.

Overall, the logical steps are relatively straightforward when taken one at a time, recognizing that leap years do not occur with the regularity we might have thought. The reason behind this irregularity is actually very intriguing.

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