Timestamp to Date in PLSQL

d_start_time_ := TO_DATE(TO_CHAR(start_time_, 'YYYY-MM-DD-HH24.MI.SS', 'NLS_CALENDAR=GREGORIAN'), 'YYYY-MM-DD-HH24.MI.SS', 'NLS_CALENDAR=GREGORIAN');

d_end_time_   := TO_DATE(TO_CHAR(end_time_,   'YYYY-MM-DD-HH24.MI.SS', 'NLS_CALENDAR=GREGORIAN'), 'YYYY-MM-DD-HH24.MI.SS', 'NLS_CALENDAR=GREGORIAN');