To have the time correct itself, you're going to have to catch every
possible combination of errors and change the time to what you think they
meant.
The input mask should prevent them from doing it wrong, and yes, it will
complain if they do it wrong. The advantage is that the input mask can also
put place holders in the textbox to guide them to doing it right to begin
with. The second step of the wizard for creating the input mask will give
you a choice of characters for the prompt. If they still insist on doing it
wrong, then after doing it wrong a couple of times they'll learn to do it
right the first time and the problem will go away.