Within a column set how to make it stay at 12/31/XX

HI,

We have a client that wants to create a column that always stays at December 31, depending on the year the Balance Sheet or P&L is run and have the first column of the report change based on current YTD or prior YTD information. Example a comparitive B/S is generated as of 3/31/24 in the first column but the second column will always be 12/31/23 Prior YTD or a report is run as of 4/30/22 and the Prior YTD column is 12/31/22. I know how to update the data source but having issues creating the date format to alawys pull as of 12/31/xx. Here is a copy of my formula what am I doing wrong? 

=Format( '{0: MMMM yyyy}',Report.GetPeriodEndDate(Report.FormatPeriod(@StartPeriod, -1, 0)))

This formula should always state 12/31/XX but it does not.

Here is my data source setting for the column set:

If I can get any guidance on how to set tht date format to reflect 12/31/xx that would be great.