You will need separate line items that contain the summarized time data before you can apply typical time functions, such as POST, on the result.
For example, when your line item is at month level and summary is quarter; applying POST at a month level does not figure at the quarter level. Would POST(Line Item, 1) mean the Quarter summary should show you:
Case 1; the previous quarter's value for that line item, or should it show you
Case 2; The summed up value at quarter after applying POST at the month level?
If Case 1: You will need a separate line item dimensioned by Quarters and then you can simply apply POST to this line item
If Case 2: Simply apply Formula for the main summary and Sum for the Time Summary (screenshot below)