Umm, I think I'd turn on "break on all errors" and figure out which line of code raises the error unless you already know. I don't see how a shortcut for entering a value would raise such an error any more than typing it in or selecting from the built in calendar control. To me, there is not enough info, like whether or not there is any code behind the form control, or exactly when the error is raised (after it loses focus? Immediately upon Ctrl+: keypress?).
IIRC, this error can be raised when a field is mis-named (EvntDay instead of EventDay, for example). Access can/will assume it's a function call; probably because it's not part of a table.field syntax ( tblMyTable.EventDay or even just because there's no brackets to flag it as a field or domain name [EventDay]).
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.