Calendar Empty (Null) date

I am using a form that saves to mySQL. It has some dates.

two questions:
a) Sometimes I don’t know the date, How can I make the calendar object write “null” to table?

b) Sometimes I’d like to type the date by myself, and I have some helper routines I wrote (like if you type 7, it fills it to the 7th of the current month(‘07/04/2012’ to the normal people!!!). So what I would like is an input box that can call a calendar IF necessary.

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