Date From  + days  add to Date to

Hi probably could find the answer somewhere, but I have spent ages on this already.
I have a form (gtsl)  which has 3 fields  fecha_entrada (date from), noches (nights) and fecha_salida (date to) The fecha_entrada (dd/mm/yyyy) is populated by a calender,  the noches field and fecha salida are both blank.
What I need is if a number of noches is entered that it automatically populates the fecha_salida in the same format  (fecha_entrada + noches) also it would be good if nothing could be manually entered in the fecha_salida

Here is what I have

<p style="margin-top: 3px; margin-bottom: 5px">Date of Arrival
            <img style="VERTICAL-ALIGN: middle; CURSOR: pointer" onclick="popUpCalendar(this,document.gtsl.fecha_entrada,'dd/mm/yyyy',220,'images/');constructCalendar();return false;" height="29" alt="calendario.gif" src="images/cal.jpg" width="27">  input type="text" name="fecha_entrada" size="9">  Nights<input type="text" name="noches" size="3">