Why do you subtract 1 in a leap year if the date falls before March?

Question

I was reading this below and I had a question: Link

Take a look under "The Key Value Method" where it says "If your date is in January or February of a leap year, subtract 1."

I understand that this is a step that we had to follow but why do we need to subtract 1? I try to find it on google but had no luck.

again my question is: why do you subtract 1 in a leap year if the date falls before March?

Answer 1

Because it will be leap year only if there is $29^{th}$ $ February$, which can not come before march (more precisely 29th feb).

So while calculating we need to subtract 1 as its not a leap year yet.