I'm doing mail merge in MS Word 2016 to create letters that I mail out. The letters have a date on them representing the date I mail the letter.

Rather than update that date every time, I want a field to insert the current date when I run the mail merge. However, when I create a set of documents by running the mail merge, I don't want that date to update if I later open the document. I want the date to remain the date that I ran the mail merge.

If I use the "Date" field, it works fine when I run the mail merge but updates the date if I later open one of the files created by the mail merge.

If I use the "CreateDate" field, it shows the date the original document was created, not the date I ran the mail merge.

What can I do to get it to automatically give me the date that I run the mail merge, but not update it later if I open a file created by the mail merge?