Override Calculated Field Script

Hello Everyone,

I have two fields on a form that are used as a start and end date for a temporary duty which is normally 10 days.  I have a script that automatically enters a date into the end date field which is 9 days after the start date field.  I would like the user to be able to override and enter a different date if the temporary duty is going to be less than 10 days.    Below is the script I entered into the End Date field:

var date= util.scand("dd mmm yy", this.getField("PTDYStart").value;

date.setDate(date.getDate()+9)

if (this.getField("PTDYStart").value!="")

{

event.value=util.printd("dd mmm yy",date)

}

else

{event.value=""}

I welcome any and all assistance with this.