This Answer may help.
Unfortunately, that referenced answer doesn’t address the question.
No, the startdate and enddate fields should be DATE or TIMESTAMP fields or be DATE expressions. You may convert various character fields into “dates” with a variety of functions.
Overall, you shouldn’t be using Query/400 at all. Use Query Manager instead. See the STRQM command. (It might not be installed. SQL options are needed, though experience can show you how to create and use QM queries even without having the SQL options product installed.)