The calendar pop-up window (to set the value in a Date Time field) behaviour is a constant source of complaints in our IT Department as the checks for an impossible date occur every time the hour or minute values are selected and not just when the final specification is made (day of the displayed month is selected).   Behaviour seems to be that you select the time first (i.e. hour, minute) because selecting the day of the month closes the edit window and enters your selections as the date field contents.  However, this means that the errors for impossible dates may appear when you select hour, then when you select minute - meaning you must 'OK' repeated errors for an impossible date before you finish selecting the date you intend.

(i.e. you must OK a message "...date must be <prior> to start time..." or similar  even if the date you intend on finally selecting makes the message invalid.)

Every time you navigate to a new month in the calendar pop-up, any unsaved hour/minute edits revert to the saved/current hour/minute and the window refreshes to it's original location if you had moved it.