When you insert some date, a popup appears while you are typing (I think it's called "ui-inputextender") and builds a meaningful date from the characters you typed. The problem is, this popup reloads as-you-type, and it takes some time to load; so if you press the Save button before the last character of the date was loaded in the popup, then a truncated date is saved (eg. 1 instead of 1999).
I think a safer behaviour would be to disable the Save button when the user types a key and the popup starts loading, and re-enable it when the popup completes.