Excel formula for subscription revenue recognition

You must be talking about forecasting accretion of revenue for contracts sitting in deferred revenue on your balance sheet. The approach I used was to create a three-tabbed spreadsheet with the following tabs:

Amortization Schedule
Reconciliation
Cumulative Amortization

The amortization schedule yields period revenue. Reconciliation yields balance sheet deferred revenue. Cumulative amortization controls for ensuring things zero out properly.

Columns in the amortization schedule should include start date, end date, as well as the amount. You're probably better off calculating a daily amortization, since that eases the computational complexities of the starting month and ending month. Other than that, it's pretty straight-forward.