Date And Time - Showing Date 90 Days Later

Say I put a date and time for like this: Jun-15-04 21:52:06. Here is the form I am using:

<form>
<p><input type="text" name="T1" size="20"><br>
date</p>
<p><input type="text" name="T2" size="20"><br>
90 days from date</p>
<p><input type="submit" value="Submit" name="B1"></p>
</form>

How could I get to show the date 90 days later to the exact date and time. I would enter the data in the first box and hit submit to show in the second box.