Dilemma with automatic Date calculation

Hello,

i've got a problem that drive me crazy.
In a XML-form I use a JS by thomp, that I found in a tutorial.
The JS works in the calculate event of a textfield in flowtext to output the expire-date of this form.
To avoid that the JS will be calculated everytime the filled form is opened again I used an IF-condition for it.

if (Formular1.Seite1.Body.Dienstleitungen.EndDatum.rawValue == null)
{
// Get todays date
var RightNow = new Date();
// Calculate todays Milliseconds
var msRightNow = RightNow.getTime();
// Calculate 7 days in Milliseconds
var sevenDays = 7 * 24 * 60 * 60 * 1000;
// Calculate final date
var finalTime = msRightNow + sevenDays;
// Convert to date
var theNewDate = new Date(finalTime);
// Output the final date
event.value = util.printd("dd.mm.yyyy",theNewDate);
}
else
{}

In the preview of Designer this works perfectly and also in Acrobat/Reader.
But I have to enabe User Rights for the form, that causes a dilemma I could not solve until now.
After enabling the rights and saving the form, the IF-condition is never NULL again and there is always an expire-date in the form that has been calculated before enabling the rights.
I have a reset-all-button in the form with that I can force the form to calculate a new expire-date, but that's no a end user friendly solution to say: "Please reset the form after opening even if it is blank".

So I need a solution.
Please help!