Date in a PDF Form Changing when PDF is Opened Next Day - How to Lockdown the Date Field?

Hi,

I am facing an issue with a PDF form that is used and signed by mutiple people. As per the process, once first person completes his form fields, he presses the First "Completed" button and the current date must be displayed against his name in a previously hidden field (I have programmed the button to make date field visible once button is pressed). Then the form goes to second person. When the second person completes his form fields, he presses the Second "Completed" button and the current date must be displayed against his name in a previously hidden field.

The problem I am facing is that I am able to get the current date, but when the PDF is opened on the next day, the date keeps changing to reflect new date. I don't want this and need to lock the date field once a date is populated for record. I have tried making the field readonly after the current date is populated first time, but still the date gets changed automatically the next day.

The current Custom Calculation Script used by me in a text field is:
event.target.value = util.printd("mm/dd/yyyy", new Date());
event.target.readonly = true;

Can anyone provide guidance on this?