Converting Dates into Character?

We are trying to get a Date field into Month / Year format in a report.  In order to get the reporting column into Month / Year format, we are converting the column into a String.  The String works, but when you try to sort on the column the report is treating the Month / Year as text (ex. the order appears as 1, 10, 11, 12, 2, 3, etc.).  Any idea on how to enable the sort to work correctly?  Below is the formula we are using.

CAST(YEAR(Indexed Date) AS CHAR) || ' / ' || CAST(MONTH(Indexed Date) AS CHAR)

Thanks!