Excel how to add the specified time to the specified time, more than the off-hours and then from the working hours

For example, I am working at 8:00:00 and 20:00:00 at work. It is now: 2018/6/6 18:36 plus 8 hours over the end of work time at 20:00:00 and then from tomorrow's working time 8:00:00 to 2018/6/7 14:36. Is there such a function formula, begging for the gods