Would you be able to re-post the workbook as a twbx file? Thanks!
Sorry about that Tracy.
I think I found a solution to the problem. I calculated a new field, called December, that is zero for every month except December, where it is set to equal profit. I then created a table calculation using that new field, called Last December, that looks back 12 months across the date field -- WINDOW_SUM(SUM([December]), -12, -1).
I still don't fully understand how the table calculation works in this context. Specifically, these relative values in the WINDOW_SUM function are looking at date fields in the pages shelf, which only works here because the months and the year are part of the same field but I found that they must be individually listed on the pages shelf.
If anyone could explain what's going on here, or point me to the appropriate documentation, I'd be grateful. I'm going to be using these types of calculation in bullet graphs to compare values (say, profit levels) to certain important values from the past (past profit, last years total profits, etc.).