Google is notoriously bad at indexing the correct time a Reddit thread is posted.

I have no clue why this is, I assume perhaps it is seeing a different date somewhere on the page (e.g. the "recently viewed threads" box).

It doesn't seem to be directional either. That is, it's not just old threads pretending to be newer, but new threads also sometimes appear older.

This is why I don't attribute any malice to the issue.