After periodically getting reports from users of date fields showing values that they'd not entered, I finally learned that someone or some team at Lotus, for some reason I've failed to understand yet, coded the date picker in the R5 and later Notes clients rather, um, strangely. For example, by default, if the user enters the letter "a" in the date field, it will be translated as April 1st of the current year. You can read a detailed description of the "One- and Two-Parameter Date Input" feature on the IBM site. OK, while I don't yet know what the motivation was to code the behavior of the date picker this way, fortunately one can modify the behavior for the user in their Lotus Notes client by adding this environment variable setting to the Notes.ini file:
 StrictDateTimeInput=1

I now need to do a lot of testing and see if setting this parameter will break anything else for my users. If the Notes.ini change raises issues, then I may experiment with writing a date picker wizard in LotusScript, since there does not appear to be a way to force one to use the date picker. Well, I guess this is part of what I like about Notes (and what drives me mad) ...that there is always more to learn :-)