Could someone please suggest some good date/time modules to compare dates or increment/decrement dates.

There are a gazillion CPAN modules for that. For simple things, I like Date::Simple.

Ideally I would to achieve this through some standard function

I think most, if not all, CPAN date modules store dates as integers, or a set of integers. Incrementing/decrementing dates doesn't use anything less standard than addition, subtraction, and numeric compare. Perhaps the modulus function if there's a need to find out whether a year is a leap year.

without using any additional module.

Whatever. If you want to reimplement one of the many existing wheels out there, be my guest. I'm not going to do that for you. You could always take the source code of the mentioned modules and use that as an inspiration.