Question

Using Oracle PL/SQL I need to calculate a start date for a task using the due date and the working hours required to finish the task.

Given 24-JUL-2012 17:00 as a due date and 20 hours as hours needed to complete the task and using working hours of 8AM-5PM (1 hour lunch – so 8 hours per day max) I need to figure out in PL/SQL what the start date/time would be… which should come out to 22-JUL-2012 13:00.

Answer 1

The following code could be a starting point:

function task_start_date(
  p_due_date date,
  p_working_hours number
) return date
is
  l_start_date date;
begin
  -- Subtract full days
  l_start_date := p_due_date - trunc(p_working_hours / 8);
  -- Subtract remaining hours
  l_start_date := l_start_date - mod(p_working_hours, 8) / 24;
  -- Fix date if the due date is before 8AM
  if to_number(to_char(l_start_date, 'HH24')) < 8 then
    l_start_date := l_start_date - 15 / 24;
  end if;
  return l_start_date;
end task_start_date;

Note that the function does not consistently consider lunch time. You’ll need to define exactly when lunch time is and adapt the function accordingly.