This is along the same lines as another post that has a lot of good discussion on the subject. There are many ways of doing this kind of thing, but I must admit to being a little biased towards my way. :) And if you read through some of the comments you'll see why.

To give a very brief overview, many of the methods deal with converting the number into epoch seconds using timelocal and comparing the dates in epoch seconds using something like $ninetyDaysAgo = time - ($numberOfDays * 86400) and compare $ninetyDaysAgo with the value returned from timelocal.

At any rate, check out that series of posts and you might find some good ideas!

Good luck,
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