Oracle 18c

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alter session set nls_territory = 'AMERICA';
with dates as (
 Select timestamp '2019-05-31 08:00:00' start_date
      , timestamp '2019-06-03 09:00:00' end_date
   from dual
)
, recur(start_date, calc_date, end_date) as (
  -- Anchor Part
  select start_date
       , trunc(start_date)
       , end_date
    from dates

  -- Recrusive Part
  union all
  select start_date
       , calc_date+1
       , end_date
    from recur
   where calc_date+1 < end_Date
)
, days as (
  select calc_date
       , case when mod(to_number(to_char(calc_date,'d'))-1,6) != 0 then 1 end isWeekDay
       , greatest(start_date, calc_date + interval '8' hour) start_time
       , least(end_date, calc_date      + interval '16:30' hour to minute) end_time

       , least( ( least(end_date, calc_date      + interval '16:30' hour to minute) 
                - greatest(start_date, calc_date + interval '8' hour)
                ) * case when mod(to_number(to_char(calc_date,'d'))-1,6) != 0 then 1 end
              , interval '8' hour
         ) daily_hrs
    from recur
   where start_date < (calc_date + interval '16:30' hour to minute)
     and (calc_date + interval '8' hour) < end_date
)
select calc_date
     , daily_hrs
     , numtodsinterval(sum( extract(hour from daily_hrs)*60*60
                          + extract(minute from daily_hrs)*60
                          + extract(second from daily_hrs)
                          ) over (order by calc_date)
                      ,'second') run_sum
  from days;


CALC_DATE
DAILY_HRS
RUN_SUM


31-MAY-19
+000000000 08:00:00.000000000
+000000000 08:00:00.000000000

01-JUN-19
null
+000000000 08:00:00.000000000

02-JUN-19
null
+000000000 08:00:00.000000000

03-JUN-19
+000000000 01:00:00.000000000
+000000000 09:00:00.000000000


alter session set nls_territory = 'UNITED KINGDOM';
with dates as (
 Select timestamp '2019-05-31 08:00:00' start_date
      , timestamp '2019-06-03 09:00:00' end_date
   from dual
)
, recur(start_date, calc_date, end_date) as (
  -- Anchor Part
  select start_date
       , trunc(start_date)
       , end_date
    from dates

  -- Recrusive Part
  union all
  select start_date
       , calc_date+1
       , end_date
    from recur
   where calc_date+1 < end_Date
)
, days as (
  select calc_date
       , case when mod(to_number(to_char(calc_date,'d'))-1,6) != 0 then 1 end isWeekDay_bad
       , case when (mod(mod(calc_date - next_day(date '2019-1-1',to_char(date '2019-01-06','day')),7),6)) != 0 then 1 end isWeekDay
       , greatest(start_date, calc_date + interval '8' hour) start_time
       , least(end_date, calc_date      + interval '16:30' hour to minute) end_time

       , least( ( least(end_date, calc_date      + interval '16:30' hour to minute) 
                - greatest(start_date, calc_date + interval '8' hour)
                ) * case when (mod(mod(calc_date - next_day(date '2019-1-1',to_char(date '2019-01-06','day')),7),6)) != 0 then 1 end
              , interval '8' hour
         ) daily_hrs
    from recur
   where start_date < (calc_date + interval '16:30' hour to minute)
     and (calc_date + interval '8' hour) < end_date
)
select calc_date
     , isweekday
     , isweekday_bad
     , daily_hrs
     , numtodsinterval(sum( extract(hour from daily_hrs)*60*60
                          + extract(minute from daily_hrs)*60
                          + extract(second from daily_hrs)
                          ) over (order by calc_date)
                      ,'second') run_sum
  from days;


CALC_DATE
ISWEEKDAY
ISWEEKDAY_BAD
DAILY_HRS
RUN_SUM


31-MAY-19
1
1
+000000000 08:00:00.000000000
+000000000 08:00:00.000000000

01-JUN-19
null
1
null
+000000000 08:00:00.000000000

02-JUN-19
null
null
null
+000000000 08:00:00.000000000

03-JUN-19
1
null
+000000000 01:00:00.000000000
+000000000 09:00:00.000000000
CALC_DATE	DAILY_HRS	RUN_SUM
31-MAY-19	+000000000 08:00:00.000000000	+000000000 08:00:00.000000000
01-JUN-19	null	+000000000 08:00:00.000000000
02-JUN-19	null	+000000000 08:00:00.000000000
03-JUN-19	+000000000 01:00:00.000000000	+000000000 09:00:00.000000000