Found these lines in the class, CL_APPOINTMENT of the method, GET_DATA:-
CONVERT TIME STAMP appointment_data-tst_from
TIME ZONE appointment_data-zone_from
INTO DATE appointment_data-date_from
TIME appointment_data-time_from.
CONVERT TIME STAMP appointment_data-tst_to
TIME ZONE appointment_data-zone_to
INTO DATE appointment_data-date_to
TIME appointment_data-time_to.
So, TST_FROM and ZONE_FROM are converted into DATE_FROM -- this is what I need.
Thanks much~