The tag-info for a tag contains some stats in the right sidebar. At least in some cases,1 it says: "created .... ago by ". (And the timestamp shows the exact date if you hover above it.)
As far as I can tell, the date shown there isn't actually the date when the tag was created - in the instances that I've checked, the date/time seems to correspond to the creation of the tag-info.
There is a rather old bug report: "created" date and user displayed on tag wiki refer to different events. It is tagged status-completed. So maybe it was marked as completed prematurely - or maybe the bug regressed.
Some random examples:
- The tag geometry on Mathematics. The tag-info says 2011-03-15, the Taxonomist badge was awarded in August 2010. The tag-excerpt was created in March 2011. On the site you can see that there were some questions with this tag in 2010: 18, 51, 59.
- The tag homology on MathOverflow. The tag-info says 2017-05-12, the Taxonomist badge was awarded in October 2009. The tag-info was created in May 2017. On the site or from SEDE you can find some questions from 2009: 415, 590, 640.
- The tag bayesian on CrossValidated. The tag-info says 2011-12-27, the Taxonomist badge was awarded in December 2010. The tag-info was created in December 2011. On the site or from SEDE you can find some posts which had this tag in 2010: 1, 22, 39.
Other related posts:
- Tag wiki Stats - "created" showing with wrong date calculation (from 2017);
- Tag wiki creation time doesn't match actual date (from 2018).
1See also: Show a tag's creator on the tag wiki page and What are the criteria for a tag creator's userid to be shown or not?
Screenshot from Mathematics Stack Exchange:
Screenshot from MathOverflow:
Screenshot from CrossValidated:
