How to sent a date in a form as string and number at the same time?

I use the calendar function to input a date in a form.

<input class="foswikiInputField" name="date" id="action_due" type="text" size="31" value="" /><img alt="" class="foswikiButton foswikiEditFormCalendarButton" onclick="return showCalendar('date','%e %B %Y')" src="%PUBURL%/System/JSCalendarContrib/img.gif" /

Now I want to send this input date as number in date-format since 1970 in another input variable like

<input id="date2" name="date2" type="hidden" value="%CALC{"$TIME(%URLPARAM{"date"}%)"}%" />

at the same time in the same form when pressing SUBMIT.

How can I do this (I am not a Javascript expert). Many thanks in advance for help!

-- ThomasSchmidt - 13 May 2013

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