I don't want it to! :-)
I'm using custom forms implementing a date field, then saving values manually to a node later. I could add an extra step preventing the value from being saved to the node, but I won't have the necessary info available to calculate whether to do this if it's an 'edit'.
Any ideas? I just dont want to save a value if the users doesnt enter anything. Seem's simple enough to me... :-S