MySQL system date change impact
Changing the system date or the system time does not change values stored in tables.

Existing values can be changed using an UPDATE statement, though.

Datetime and timestamp columns behave differently on retrieval.

create table tz_test (
  ts timestamp not null,
  dt datetime not null
);

insert into tz_test values (current_timestamp, current_timestamp);

select * from tz_test;

ts                   dt
--
2013-06-27 09:46:14  2013-06-27 09:46:14  


Set a session timezone, and compare.

set time_zone = '-8:00';
select * from tz_test;

ts                   dt
--
2013-06-27 05:46:14  2013-06-27 09:46:14  
           ^^


Timezone support in MySQL

Docs show you how to change both client and server timezones for testing. You have to read that.