Are these dates always formatted as MMM YYYY (Apr 2010, May 2009)? Is it possible that you might have different formats?
If not, you can convert that to a datetime column either in your query or in a computed column on the table. To convert to a datetime, you would use the following:
Select convert(datetime, 'Apr 2010', 107);
Select convert(datetime, 'Mar 2009', 107);
Select convert(datetime, 'Jan 2011', 107);
Select convert(datetime, 'Jun 2010', 107);
If you have different formats, you need to identify all of the various formats and structure them (if needed) into a valid date format that can be converted to a datetime.