Using Auto Date Update in Word/merging

I merge documents a lot in Word, but I have to keep typing the date, either in the data file or main document. I tried using the auto date update but the problem with that is it will keep updating the date even after the documents have been merged, which doesn’t help me as I have to keep these documents electronically and they have to have a permanent date on them that won’t update. Anyway to do this? thanks