I am guessing tou are refering to the post times shown on the board index, the last post column. These times are stored inside the forums table, and you’ll have to adjust those aswell. (If the last post is actually one of those Announcements.
You might have to check the topics table for first and last post data too.
Otherwise, if you are viewing the topic from within ‘viewtopic.php’ and you do not see the correct time, then you have not adjusted the times in the posts table correctly.
First and last post information are often stored in other columns aswell, so information can be displayed without having to make addiotonal queries to different tables.