How to account for leap years?

Question

I have some doubts about the leap years, how can I be sure that by using a formula like this

add.years= function(x,y){    
if(!isTRUE(all.equal(y,round(y))))         


        

Answer 1

A year is a leap year if:

• Is divisible by 4.
• Not if it is divisible by 100.
• But is if it is divisible by 400.

That is why 2000 was a leap year (although it's divisible by 100, it's also divisible by 400).

But generally, if you have a library that can take of date/time calculations then use it. It's very complicated to do these calculations and easy to do wrong, especially with ancient dates (calendar reforms) and timezones involved.