Немного упрощенная версия:
select x_gsa_arrive_onsite
, x_gsa_leave_site
, trunc((x_gsa_leave_site - x_gsa_arrive_onsite) * 24) ||':'||
trunc(mod((x_gsa_leave_site - x_gsa_arrive_onsite) * 24 * 60, 60)) as onsite_hhmi
from table_x_gsa_time_stamps ts;
дает
X_GSA_ARRIVE_ONSITE X_GSA_LEAVE_SITE ONSITE_HHMI
------------------- ------------------ -----------
01-SEP-2018 01-SEP-2018 12:34 12:34
01-SEP-2018 02-SEP-2018 01:23 25:23