mysql> explain select calltime  as acd from cdrs where start >= '1432771200' and start <= unix_timestamp('2015-05-28 23:59:59') and station = 10;
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key   | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
|  1 | SIMPLE      | cdrs  | range | start,station | start | 27      | NULL | 5558 | Using where |
+----+-------------+-------+-------+---------------+-------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select calltime  as acd from cdrs where start >= unix_timestamp('2015-05-28 02:00:00') and start <= unix_timestamp('2015-05-28 23:59:59') and station = 10;
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
|  1 | SIMPLE      | cdrs  | ALL  | start,station | NULL | NULL    | NULL | 1859209 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)