First off, I have to ask; why would you have a memo field with only a date in it? You can make your life easier if you can switch this to a DATE/TIME type field.
If you can't make this switch, then consider using a type conversion function to force the type of field you want (i.e. string, date, whatever). This has worked for me in the past when I wanted a date passed as a string, not as a date (long story why I needed it to in that situation).