calculate payment with first payment due date variable?

I need to be able to calculate a payment with the ability to input the first
payment due date as a variable. for example due date would be in 45 dates
from current date.

Jody,
If the current date is in A1 from a key-in, reference from another cell or using NOW(), then in B2 place =A1+45. This will show a calendar date 45 days after the date shown in A1. Should A1 change, B1 will change accordingly. Be sure the cells are formatted the same from the many Date choices in the Format drop-down.
I hope I interpreted your problem properly. GeorgeF