Every four years (or so) we get an extra day in February, leap year. When I was a kid this term confused me. Frogs leap, they leap over things. A leap year should be shorter! Obviously, I was wrong.

This extra day can give us extra time to complete tasks (e.g. write blog post), so we are going to use our shells to check if the current year is a leap year.

PS C:\> [DateTime]::IsLeapYear(2014)
False

Sadly, this year we do not have extra time. Let's confirm that this command does indeed work by checking a few other years.

PS C:\> [DateTime]::IsLeapYear(2012)
True
PS C:\> [DateTime]::IsLeapYear(2000)
True
PS C:\> [DateTime]::IsLeapYear(1900)
False

Wait a second! Something is wrong. The year 1900 is a multiple of 4, why is it not a leap year?

The sun does not take exactly 365.25 days to get around the sun, it is actually 365.242199 days. This means that if we always leaped every four years we would slowly get off course. So every 100 years we skip the leap year.

Now you are probably wondering why 2000 had a leap year. That is because it is actually the exception to the exception. Every 400 years we skip skipping the leap year. What a cool bit of trivia, huh?

Hal, how jump is your shell?

I should have insisted Tim do this one in CMD.EXE. Isn't is nice that PowerShell has a IsLeapYear() built-in? Back in my day, we didn't even have zeroes! We had to bend two ones together to make zeroes! Up hill! Both ways! In the snow!

Enough reminiscing. Let's make our own IsLeapYear function in the shell:

function IsLeapYear {
    year=${1:-$(date +%Y)};
    [[ $(($year % 400)) -eq 0 || ( $(($year % 4)) -eq 0 && $(($year % 100)) -ne 0 ) ]]
}

There's some fun stuff in this function. First we check to see if the function is called with an argument ("${1-..."). If so, then that's the year we'll check. Otherwise we check the current year, which is the value returned by "$(date +%Y)".

The other line of the function is the standard algorithm for figuring leap years. It's a leap year if the year is evenly divisible by 400, or divisible by 4 and not divisible by 100. Since shell functions return the value of the last command or expression executed, our function returns whether or not it's a leap year. Nice and easy, huh?

Now we can run some tests using our IsLeapYear function, just like Tim did:

$ IsLeapYear && echo Leaping lizards! || echo Arf, no
Arf, no
$ IsLeapYear 2012 && echo Leaping lizards! || echo Arf, no
Leaping lizards!
$ IsLeapYear 2000 && echo Leaping lizards! || echo Arf, no
Leaping lizards!
$ IsLeapYear 1900 && echo Leaping lizards! || echo Arf, no
Arf, no

Assuming the current year is not a Leap Year, we could even wrap a loop around IsLeapYear to figure out the next leap year:

$ y=$(date +%Y); while :; do IsLeapYear $((++y)) && break; done; echo $y
2016

We begin by initializing $y to the current year. Then we go into an infinte loop ("while :; do..."). Inside the loop we add one to $y and call IsLeapYear. If IsLeapYear returns true, then we "break" out of the loop. When the loop is all done, simply echo the last value of $y.

Stick that in your PowerShell pipe and smoke it, Tim!