I would recommend using the MonthStart function combined with the Date function. This will keep field as a date and give you the display you want (you can even display months with words). Typically I do this in the script, here is a sample
Date(MonthStart(DATE_COLUMN),'YY-MM') as Year-MonthNo, //this will give you 14-02 format
Date(MonthStart(DATE_COLUMN),'YY-MMM') as Year-MonthNo, //this will give you 14-Feb format
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