Setting fixed_date to a date/time value overrides the value that sysdate (but not systimestamp) returns. In order for sysdate to return the correct value again, fixed_date must be set to none.
Unfortunately, fixed_date can only be set for the entire system, not for a particular session.
select
sysdate,
systimestamp
from
dual;
alter system set fixed_date = '0102-03-04 05:06:07';
select
sysdate,
systimestamp
from
dual;
alter system set fixed_date = none;
select
sysdate,
systimestamp
from
dual;