You can fix this by selecting the errant date, pressing Shift-F9, changing 'DATE' in the field to 'CREATEDATE' and pressing F9 to update the field. This will restore the date the document was first saved - or whenever it was most recently saved via SaveAs and will remain fixed until the next SaveAs is done.
If 'CREATEDATE' doesn't return the date you're after, you might obtain it via 'PRINTDATE' or 'SAVEDATE'. With either of these, though, the date will change whenever the document is reprinted or resaved, respectively. In that case, having updated the field you might want to lock it, via Ctrl-F11, or convert it to text, via Ctrl-Shift-F9.
To see how to do just about everything you might want to do with dates in Word, check out my
Microsoft Word Date Calculation Tutorial at the top of this forum:
https://www.msofficeforums.com/word/...-tutorial.html