Just tried it. After hitting Save button the Edit page behaved like everything is OK, and showed confirmation message that it's saved, but it wasn't OK - that's the problem I'm talking about. The page should verify the the input even before saving data, reject invalid ones, abort the save action and point out where is the problem.
In better version, the page shouldn't allow to insert invalid data - in this case show some popup calendar where user can select date. However this approach has little problem, it's not as fast as typing or copy & pasting it.