Go to File > Options > Schedule tab and define the calendar options. make sure hours per day is set at 10.



Then go to Project > Project Information and make sure the time part of your Project Start date is 06:00



After the above settings, re-enter your durations.

Microsoft Project stores durations in hours. Though you entered 8 days, the system converted it to 64 hours (8 days x 8 hours) therefore when you changed "hours per days" to 10, the system showed you 6.4 days ( 64 hrs / 10 hrs).

But once "hours per day" is set as 10 hrs, any duration you enter in days will be converted to the right duration in hours.