Where is the script located?
The script is located in the text field properties/ calculate/ and customs calculation script section
That means it will also launch when the value of any other field is changed. If you want it to launch whenever the file is opened then you need to change the last line to:
this.getField("Field Name").value = util.printd("dd/mmm/yyyy", oNow);
Thanks once again, it worked perfectly!!!!