how to partition a table by datetime column?

I want to partition a mysql table by datetime column. One day a partition.The create table scripts is like this:

CREATE TABLE raw_log_2011_4 (
  id bigint(20) NOT NULL AUTO_INCREMENT,
  logid char(16) NOT NULL,
  tid char(16) NOT NULL,
  reporterip char(46) DEFAULT NULL,
  ftime datetime DEFAULT NULL,
  KEY id (id)
) ENGINE=InnoDB AUTO_INCREMENT=286802795 DEFAULT CHARSET=utf8
PARTITION BY hash (day(ftime)) partitions 31;

But when I select data of some day.It could not locate the partition.The select statement is like this:

explain partitions select * from raw_log_2011_4 where day(ftime) = 30;

when i use another statement,it could locate the partition,but I coluld not select data of some day.

explain partitions select * from raw_log_2011_4 where ftime = '2011-03-30';

Is there anyone tell me How I could select data of some day and make use of partition.Thanks!