The 13th accounting period is typically used for entering year-end adjustments and is generally set up as the last day of the fiscal year.
For example: If the fiscal year runs the same as the calendar year (January 1 through December 31), Periods 1 through 11 would be set up for the entire months of January through November. December 1st through December 30th is period 12, and December 31st is period 13.
If any year-end adjustments need to be entered, they are entered with a Post Date of 12/31/YYYY so that the reports will show all of the adjustment activity in the same period. Since the adjustments still fall within the fiscal year, their net/surplus/deficit will also roll into the fund balance account for the beginning of the next year.