Don't forget that calendar math is not commutative!

    Jan 27 + 5 days = Feb 1

    Feb 1 + 1 month = Mar 1

    Jan 27 + 1 month = Feb 27

    Feb 27 + 5 days = Mar 4 (or Mar 3 in a leap year!)

Therefore...

    Jan 27 + 5 days + 1 month = Mar 1
    Jan 27 + 1 month + 5 days = Mar 1

AND

    Jan 27 + 5 days + 1 month = Mar 4 (or 3)
    Jan 27 + 1 month + 5 days = Mar 4 (or 3)

Before someone claims that this is just a February problem, please first consider that you can create these scenarios with any set of months that have differing lengths.

Then, please consider that your argument that ignoring February is not a good way to approach any calendaring library....