You may be able to use Chris Woodman's old Add-In to generate the field you want. You can find a link to the archival version of it here.

The tutorial you already have is a far better source of a robust field but, as you've discovered, editing these fields can be troublesome. I strongly recommend working your way through the tutorial if you need to work with these fields.

Calculated Dates in Microsoft Word using Fields or vba
Using Date Fields in Microsoft Word