How can I turn off automatic date update on opening documents

The instructions are the same for letters and for templates, but if you are basing your letters on a template that includes a DATE field, then make the change in the template and you'll never have to do it in a letter again.

The reason that the date updates is that it is a DATE field, which always provides the current system date. What you want is a different field, the CREATEDATE field, which is updated once, when you create the document, and then stays the same forever after. You can read more about these and other fields at http://wordfaqs.mvps.org/datefields.htm.

To change your DATE field to a CREATEDATE field, press Alt+F9 to display the field code. Then just type CREATE before DATE, press Alt+F9 to toggle back, and press F9 to update.

If you put a CREATEDATE field in a document template, the template itself will always display the date it was created, but when you create a new document based on the template, the document will display the date IT was created. The nice thing about this is that the date that the document was created is stored within it at creation, so you can change a DATE field to CREATEDATE at any time to recover that original date even if the DATE field has updated to the current date.

How about this? Select date, right click, check "Preserve date". But that would be too simple, and there would be less need for de-obfuscators.

And how, exactly, does this help someone who has a document, created on a different day with a DATE field? Are you suggesting a change to Word rather than explaining to someone who has a problem how to fix it?

Preserve date might be less cryptic in this instance than "Unlink field," but it does not give you the original date. Assume a document written with a DATE field on October 23, 2009, saved as a new document on July 11, 2010, and today is August 13, 2012. What date is to be preserved? How do you tell the computer?

The best method, as I believe Suzanne mentioned is to have a template using the CreateDate field to start with. The next best solution is to manually change the field to a CreateDate field or unlink the field when the document is created so that it is plain text. Going backwards is always going to be a bit iffy, whether done manually or by some right-click menu choice.