New SG pledges may get benefit expiration dates that are 13 months in the future instead of 12 months. Can occur when Sustainer Renewal Basis parameter is set to 'Rolling" and "Should Expiration Dates Be First or Last of Month" parameter is set to 'L' for last of the month.
This may occur if the Current_Start_Date for the Activity Type is set to the end of the current month instead of the end of the previous month. When your Expiration Dates are set for the end of the month, you want the Current_Start_Date to be the previous month. You only want it the current month if it is set to the first of the month. So if it is April, you want the Current_Start_Date to be 3/31, not 4/30.
The Current_Start_Date is updated by the procedure Update Current Start Date, which can be either run stand alone from schedule admin or as an output procedure with a query and output. You run this process near the end of the month, after you have sent out the last of your benefits for the month. When you have your Expiration Dates set for the end of the month, you want to run this job with a "New Start Date" parameter of just 'T'. This will update the Current Start Date to the end of the current month. The default setting for this is "T+1M", which is intended for use when Expiration Dates are set for the first of the month, to set it to the first of the next month. If you run "T+1M" when you are using end of month Expiration Dates, then it will set Current Start Date to the end of next month, which results in benefit end dates being set 13 months out instead of 12.