Activity log entry before registration date
I looked at the last page of my activity tab today, and discovered it is full of
created TAGNAME description
created TAGNAME excerpt
entries. So are the two pages before it.
It seems that for every tag wiki I edited, I am credited with creating that excerpt or description on the date it was originally created.
For example, I joined SO in December 2010, and the hover text in the above screenshot clearly shows I created the java wiki excerpt months earlier.