SQL & PL/SQL

I am partitioning a table in the following way, but i am receiving the error:

ERROR at line 13:
ORA-14019: partition bound element must be one of: string, datetime or interval literal, number, or MAXVALUE

what is the reason, sysdate can be used with numeric values then what is the problem, how can this be corrected.

CREATE TABLE ERG_KWH_LOG(
erg_kwh_log_id number(10) not null,
erg_device_id number(10) not null,
tstamp date not null,
count number(10),
kwh number(10),
demand number(10),
created_by number(10) not null,
date_created date not null
)
PARTITION BY RANGE(tstamp)
(
PARTITION erg_kwh_log_past VALUES LESS THAN(TO_char(sysdate-180)),
PARTITION erg_kwh_log_2008_jan VALUES LESS THAN(TO_char(sysdate-365)),
PARTITION erg_kwh_log_2008_feb VALUES LESS THAN(TO_char(maxvalue))
);

Peeks