Put a -1 in the Get future time interval.

Or, if you have a specific date you want to calculate, use this expression
subtractFromTime(utcNow(),1,'month')
This example subtracts 1 month from utcNow(). If you have a different date value, say, something you took from a SharePoint list or an Excel table, then use that value in place of where utcNow() appears in the expression.
If this helped, please mark it as a Solution.
Anne