percona mysql5.5.29 index on timestamp will not use

I have a table defination like this:

CREATE TABLE `t_pearl_finance_detail` (
  `fin_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `partner_id` int(11) NOT NULL,
  `createtime` timestamp NULL,
  PRIMARY KEY (`fin_id`),
  KEY `createtime` (`createtime`),
  KEY `partner_id_2` (`partner_id`,`createtime`)
) ENGINE=InnoDB AUTO_INCREMENT=36023268 DEFAULT CHARSET=utf8

mysql> explain select * from t_pearl_finance_detail force index(createtime) where createtime>from_unixtime(1364486400)\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_pearl_finance_detail
         type: range
possible_keys: createtime
          key: createtime
      key_len: 5
          ref: NULL
         rows: 3504312
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from t_pearl_finance_detail where createtime>from_unixtime(1364486400)\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_pearl_finance_detail
         type: ALL
possible_keys: createtime
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 9220001
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from t_pearl_finance_detail where createtime>'2013-03-30'\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_pearl_finance_detail
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 8732399
        Extra: Using where
1 row in set (0.00 sec)

mysql> explain select * from t_pearl_finance_detail force index(createtime) where createtime>'2013-03-30'\G;
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: t_pearl_finance_detail
         type: ALL
possible_keys: NULL
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 8732539
        Extra: Using where
1 row in set (0.00 sec)

I wonder why above sql will not use the 'createtime' index?