By Word temp/lock file I presume you are referring to the file that starts with "~$", and it is this file that is not deleted when the Word document is closed without saving, as it should be.
Sometimes these ~$ files become orphaned. Have you tried deleting it after closing the original document.
Also does this happen with any other documents. If you create a Word document, save & close it, reopen it, modify it and close it without saving, is there a ~$ file for it.
If this file remains only for the file with the date field, try copying it's contents to a new Word document, save it with a new file name and see if the problem has been resolved.
If the problem appears to be affecting all documents, then consider a Word repair.