Use @monthnum in @if statements to model one-time-a-year occurrences. Strategic Finance numbers months from 1 to 12 beginning with the first month of the fiscal year. For example, the function is used if a liability were paid out on a month.
If income Taxes Payable is v2530, Total Taxes is v1690, and September is month number 9, this formula:
@if(@monthnum= 9, v1690, v2530(-1m)+v1690)