It is possible to search for who added comments to Bugzilla and when they did it. In boolean charts,
"Commenter" "is equal to" "<person>"
will find bugs to which that person has added a comment.
"A Comment" "changed before" / "changed after" "<date>"
finds bugs where a comment was added before or after a certain date. You can combine the two (must be in the same boolean chart in order to apply to the same comment) to look for bugs where a particular person added a comment before or after a certain date.
Example: all bugs you have commented on since June 18th 2010 (also uses pronoun substitution)