I suspect by now you've figured out how to do this. But I just had the same problem and I all I found was this unanswered posting. I figure others will have it in the future. So here's the answer I came up with.
For todays I use
SELECT * FROM my_table WHERE post_time > $today AND post_time < $tomorrow
For all before postings before a certain date
SELECT * FROM my_table WHERE post_time < $before
I use PHP to generate the $before, $today, and $tomorrow variables (in format YYYYMMDD).
You could probably use the mysql date_add, date_sub, and now functions to generate the variables instead of PHP, but I'll let someone else figure it out.
