Are you sure you want a watermark?
Reader cannot add the OCG watermark layer!
So one could combine the create watermarkFromText and compute the text string for the date and time from the system date and time or form info creation date or modified date.
Have you considered using read only form fields?
And what do you mean by read only form fields?
Form fields can accept user input or locked by setting the field's property to read only so only calculations or scripts can update the data.
A PDF watermark is like the watermark on real paper stationary, it is permanent and can not be updated.
There are many sites with tutorials about scripting PDFs. It is far to much to post in a forum post.