The point is that "Today" or "Yesterday" is more instantly intuitive than a date for users and for Moderators who are checking forum's posts all the time.
This is the reason why it's default in VBulletin's forums and in many others too.
Consequently, we prefer date format with Today/Yesterday for the last two days, it will be easy to determinate last active topics without decoding and remembering today date .... some members might not like the new format too.