…this post was published on 2016 March 31.
The last 2 steps are those for calculating hours in first and last day.
Let’s begin with the first day.
As I said in main post, we assume that the working day begins at 8.30am and stops at 16.30 in the afternoon. We don’t take care for lunch break. 🙁
This mean that for a full day we have 8 working hours.
We need first, 2 new columns, calculated columns, which will calculate for StartDate day working hours.
The names for these 2 calculated columns, as DateTime including Time, are
FirstWorkingHour_FirstDay
LastWorkingHour_FirstDay
The formula to calculate them are
FirstWorkingHour_FirstDay=DATE(YEAR(StartDate);MONTH(StartDate);DAY(StartDate))+(60*8+30)/24/60
LastWorkingHour_FirstDay=DATE(YEAR(StartDate);MONTH(StartDate);DAY(StartDate))+(60*16+30)/24/60
I the first one, (60*8+30)/24/60 represent the calculation for 8.30 am. If in your case the working days starts at 10.00am.
You will use this:(60*10+0)/24/60
In the second one (60*16+30)/24/60 represent the calulation for the end working time: 16.30. You can change the formula and for 17.30, you can use this formula (60*17+30)/24/60
Attention. If your work interval is diff than 8 hours, you should change that values in the formula in case ii!
Because we don’t use validation, limitation for StartDate and EndDate columns, we should have 3 situations
i ) StartDate’s hour is less than 8.30 in the morning
ii ) StartDate’s hour is between 8.30 am and 16.30
iii ) StartDate’s is more than 16.30
More sintetic, the values will be:
case i)  case ii)  case iii) 
540 minutes ( 16.308.30) 
1440*(LastWorkingHour_FirstDayStartDate)

0 minutes 
We create a new calculated column, with the name: WorkingHours_FirstDay as numeric with 2 decimals
The formula for this will be:
WorkingHours_FirstDay =(IF(1440*(LastWorkingHour_FirstDayStartDate)>0;IF((FirstWorkingHour_FirstDayStartDate)>0;540;1440*(LastWorkingHour_FirstDayStartDate));0))/60
BUT , we don’t have working hours Saturday and Sunday, this mean that we need to eliminate these values for such of days
So the complete formula for WorkingHours_FirstDay is
WorkingHours_FirstDay =(IF(OR(WEEKDAY(StartDate)=1;WEEKDAY(StartDate)=7);0;IF(1440*(LastWorkingHour_FirstDayStartDate)>0;IF((FirstWorkingHour_FirstDayStartDate)>0;540;1440*(LastWorkingHour_FirstDayStartDate));0)))/60
The results are:
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