mysql> select now();
+---------------------+
| now() |
+---------------------+
| 2009-01-31 21:54:36 |
+---------------------+
1 row in set (0.00 sec)
mysql> select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
| 1233435200 |
+-----------------------+
1 row in set (0.00 sec)
mysql> select from_unixtime(1233435200);
+---------------------------+
| from_unixtime(1233435200) |
+---------------------------+
| 2009-01-31 21:53:20 |
+---------------------------+
1 row in set (0.00 sec)
mysql> select count(*) from sessions where timestamp > unix_timestamp(now() - interval 31 day);
+----------+
| count(*) |
+----------+
| 8711 |
+----------+
1 row in set (0.06 sec)
mysql> select count(*) from sessions where timestamp > unix_timestamp("2009-01-01 00:00:00") and timestamp < unix_timestamp("2009-01-31 00:00:00");
+----------+
| count(*) |
+----------+
| 8407 |
+----------+
1 row in set (0.08 sec)
more info: http://dev.mysql.com/doc/refman/5.1/en/date-and-time-functions.html