javascript?

I need to run the code below if a date is placed in txtBidOpenDate.Text. I having it working on the VB code side but it doesn't fill in the textbox till after the trip to the server. I need it to be filled in on the client side...

Code:

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'txtIntDisEndDate.Text = DateAdd(DateInterval.Day, 1, CDate(txtBidOpenDate.Text))

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<Script language="javascript">
''This will put todays date in ...
'txtIntDisEndDate.Text = new Date()

</Script>

how do I test if a date was entered in the other textbox?