Well, I'm not clear on the question, but, by reputation, I'm kinda dense.
In PHP, generally I use [man]date/man and create some construction with, for example, [man]strtotime/man, something like:
<?php
echo (date('r',strtotime("yesterday")));
?>
But, it sounds like you may be asking for something a little more database oriented. I play with MySQL some, and just found this:
mysql> select current_date;
+--------------+
| current_date |
+--------------+
| 2006-12-23 |
+--------------+
1 row in set (0.05 sec)
mysql> select current_date+1 day;
+----------+
| day |
+----------+
| 20061224 |
+----------+
1 row in set (0.04 sec)
mysql> select current_date-1 day;
+----------+
| day |
+----------+
| 20061222 |
+----------+
1 row in set (0.00 sec)
Either of those worth anything to ya?
Merry Christmas (or whatever holiday, if any, is up for you)!