Lots of assumptions being made here
=FV(0.04,(TODAY()-A2)/365,0,-B2)
..04 = assumed annual interest rate
Today() = is the "as of" date of the calculation - this probably needs to be
changes
a2 = date payment
b2 = the amount of payment
Then sum up all the individual accumulations. using sum(range) - example
=sum(c4:c300)
There are several different "calculation" methods that could be used - this
is a common one.