The Stamp Tool automatically updates the date and time when you publish to a pdf. Besides the Date and Time it also shows the File Name but if you just need the date (as I do), go into the Properties and deselect everything except the Date. Works great. Works, also when printing or exporting to a pdf.
As mentioned in previous post:
"When you publish to a pdf, you need to make sure that the PDF Export options have "Reset all Plug-in objects that require a reset......" checked."