I have a number of documents that I have worked on over several years and printed and saved as PDFs at various times. It essential to know that I am using the most recently saved versions and that the date fields only update when saved, not when the document is opened.

If you insert date and time fields in a document using Insert > Date & Time on the ribbon that will insert Date and Time fields that update to the current date and time when you reopen the document.

Instead, you need to use Insert > Quick parts > Field > SaveDate. That will insert the date on which the document was last saved. You just need to be careful, if you haven't edited it, to close without saving.

If you insert SaveDate and Time fields the first will show the last saved date, the second the current time, NOT the last saved time. You need put both in the SaveDate field. The format options offered didn't give the one that I wanted so I typed it in the dialogue box as d MMMM yyyy HH:mm. (Americans would use MMMM d, yyyy am/pm). When reopened the fields show the date and time when last saved, NOT the current date and time.