particular field type not allowed to this type of partitioning

[Dhileepan M]

I need to create the table in the below format. But is returning the error in the partitioning area that is

Error Code: 1659.Field 'fldassigndate' is of a not allowed type for this type of partitioning. How to resolve this error and make partitioning ?

CREATE TABLE tblattendancesetup (
fldattendanceid int(11) NOT NULL AUTO_INCREMENT,
flddept varchar(100) DEFAULT NULL,
fldemployee varchar(100) DEFAULT NULL,
fldintime varchar(20) DEFAULT NULL,
fldouttime varchar(20) DEFAULT NULL,
fldlateafter varchar(20) DEFAULT NULL,
fldearlybefore varchar(20) DEFAULT NULL,
fldweekoff varchar(20) DEFAULT NULL,
fldshiftname varchar(20) DEFAULT NULL,
fldassigndate varchar(20) DEFAULT NULL,
fldfromdate varchar(20) DEFAULT NULL,
fldtodate varchar(20) DEFAULT NULL,
fldrefid varchar(20) DEFAULT NULL,
UNIQUE KEY fldattendanceid (fldattendanceid),
KEY in_attendancesetup (fldemployee,fldintime,fldouttime,fldlateafter,fldearlybefore,fldfromdate,fldtodate,fldattendanceid),
KEY i_emp_tmp (fldemployee),
KEY i_emp_attendance (fldemployee)
)
PARTITION BY RANGE (fldassigndate)
(PARTITION p_Apr VALUES LESS THAN (TO_DAYS('2012-05-01')),
PARTITION p_May VALUES LESS THAN (TO_DAYS('2012-06-01')),
PARTITION p_Nov VALUES LESS THAN MAXVALUE );