Calculating monthly average in a pivot table of my choosing, not current month we are in.

I have a report built out that includes a pivot table that is pulling monthly averages of the prior two months for both a numeral and a money value. This is always reported for the previous month.

For instance, in March I will send a report including data for January and February only and want to show an average cancels that occurred in those months (total number divided by two). The problem I am running into is that the average just automatically calculates for whatever month I am currently IN, not the months that have data filled out. So if I open it in March, it will divide it by 3. If I open it in July, it will divide it by 7.

Can anyone help me? I'm not the person who created the document so I am not finding anywhere to fix this.

thanks!!