You appear to high jacking the OP's post.
You have to identify the products the users filling in the form will be using. Only Acrobat or maybe some other full feature editing program can dynamically change a script using the setActiion or addScript methods.
That means you need to figure out another technique to prevent the update. The easiest test is to only change the date field if it is empty. But this means the saved copy of the form that is distributed must have the date field empty when distributed.
You do not want users to use Apple's Preview application!
If there are mobile devices involved, you need to test on each platform and with each app that will be used because not all mobile device apps support JavaScript within PDF forms and many do not support forms at all.
var cDateField = "MyDateField";
if(this.getField(cDateField).value == "") {
// date field is empty so we fill it in;
cDate.value = util.printd("d-mmm-yyyy", new Date());
} // end fill-in date field;