For some reason (and I'm sure there is one) all 3347 comments posted to my site prior to the 2.661 to 3.01D upgrade are dated July 23, 2004 at 10:46 a.m.
I checked my Db, and what appears to have happened is this: the 'comment_created_on' and 'comment_modified_on' were wrong. The former showed today's date, and the latter the date the comment was actually submitted.
For the hell of it, I reversed the two field names, and left the type, attribute, et al as they were. My comments are now all properly dated.
I have no idea what effect this will have, but I posted a comment and it worked fine.
I just entered my TypeKey token, and am rebuilding (for the dozenth time today) - we'll see what happens now...