Code Help Needed

I have this code in a PHP form. Is it possible for me to add some jscript so when I type in 08/29/2011 in the sent_date field & click the calculate button, the “Due to Reviewer” field automatically populates a date -42 days from todays date?

<b>Sent Date: MM/DD/YYYY </b><br><input type="text" name="sent_date" size="15" maxlength="30" value="'.$row[8].'" /> 


<p><input type="button" value="Calculate" onclick="return dcheck(this.form);">	

<b>-----------------------------------------------------------</b> <br /><br>

<b>Due to Reviewer:</b><br><input type="text" name="due_rev" size="15" maxlength="30" value="'.$row[10].'" /> <br>

are you looking to incorporate it in the function already occuring when you hit calculate?

return dcheck(this.form);