let me understand your question please.
what i understand is as follows:
you have a ready made calendar code of javascript.
you select say 2 - 2 - 2010 from calendar and it should be updated in the text box. right?
if this is the question then, according to my understanding you will have to convert the date format into text and send it to the server in the url
string and may be retrieve it using getParameter().
i may be wrong but this is what comes to my mind.
thank you.