Date based expressions showing as text fields

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I've created a report for a client which has pivot tables in it. I am grouping results by month and then week.

I have used the following expression to display the month from the interview date: DATE_FORMAT([REQU|REQU-REQU_CAND|REQU_CAND-INTV|date], "MMMM")

The problem I'm facing now is that the resulting expression field is a text field, so when I try to sort the column to show the months in order, it is displaying them in alphabetic order. For example, the report for the last two months displays as April, June, May

Has anyone else encountered an issue like this? I'm open to suggestions...