Yes you should convert them to a date field and also use the date diff at the same time.
Not sure what your date strings look like so I cannot tell you how to convert them but one option would be something like:
datediff("d",cdate({table.receiveddatefield}),cdate({table.closedatefield}))
Note that this is checking for the number of days between the 2 dates (the "d" in the datediff). If you need a different interval change it accordingly.
Edited by DBlank - 23 Feb 2009 at 2:48pm