Hi,
When I enter a date in Word for mac at work I'm careful to check the "update automatically" checkbox isn't marked. I check every couple of days that dates haven't been annoyingly updated to the "today" date in the mac.
At home I have Word 2013 and also, when I enter a date I check "update automatically" checkbox isn't active.
Surprisingly, today I noticed all my dates have been updated to "today" again!.

Note: Sometimes the word document at home with windows 8 and word 2013 asks me to update to the newer docx format and I answer yes. May this have something to do with this?