if you just wanna do the month; you can do two things first.

( CASE [fldmonth]
           WHEN 'January' THEN 1
           WHEN 'February' THEN 2
           WHEN 'March' THEN 3
           WHEN 'April' THEN 4
           WHEN 'May' THEN 5
           WHEN 'June' THEN 6
           WHEN 'July' THEN 7
           WHEN 'August' THEN 8
           WHEN 'September' THEN 9
           WHEN 'October' THEN 10
           WHEN 'November' THEN 11
           WHEN 'December' THEN 12
        END )


otherwise, CAST('{Date}' AS DATETIME) and you can work with datediff