Replace the date (in quotes) with your start date and
100 with your amount that will serve as both the initial amount and the increment.
If you want a more accounting-accurate version:
FV is Future Value (of an investment). Here,
0 is the interest rate,
DATEDIF will figure out how many complete months have passed as marked from the date in quotes, the first
-100 is the payment amount (which, in accounting, is always negative), and the second
-100 is the initial value contributed on the start date. Using this method would allow you to set different values for the initial value and the monthly increase thereafter.