Dates being the arbitrary and complex things that they are, I'd abandon trying to be elegant and just go with the big ugly obvious approach.

You have the days already submitted, so just add more days, check for month overlap etc.. Don't convert to seconds past epoch at all. This might miss weird leapyear stuff, but it will work %99.99 of the time and it will be easy to maintain because it will be really obvious what it's doing.

--Pileofrogs