Show the format of your string. It may be smarter to do so using a SQL Expression, but that depends upon your version of Crystal and the database.
A one line post conveys very little, please take the time to share meaningful technical information.
In general you can use the DATE function, such as:
whileprintingrecords;
stringvar MyDate:= "12/31/2006";
date(MyDate)
or if you have a format Crystal won't implicitly convert, use an array function, such as:
whileprintingrecords;
stringvar Array MyDate:= split("12/31/2006","/");
date(val(MyDate[3]),val(MyDate[1]),val(MyDate[2]))
-k