First off, don't use a textbox for Date / Time entry unless you have no alternative: they are a PITA as the user can enter what he wants, in whatever format he wants, and it may not be correct. For WinForms, use a DateTime picker instead - it's more friendly for the user, and it can't generate a "bad" DateTime value.
But if you must, then look at
DateTime.TryParse[
^] which lets you check (and convert if possible) a string to a DateTime value.
Once you have the DateTime, you can pass that directly to SQL via a parametrized query.