How do I use DATEDIFF to count only working days?

I need a bit help with working days in Microsoft Project. I am trying to subtract two dates (Start date and Finish date) in order to get the number of days between them. However i am not able to do the same, the dates include Saturdays and Sundays as well. I have tried using this DateDiff("d",[Start],[Finish]) and i have tried using "ww" instead of "d" but no luck. Is there anyone here who can help me out???

Don't use DateDiff, instead try using ProjDateDiff.

You can try below formula, If you are using Number field then you will have to get number as output for example "1" or "2". If you are using Text field then you can get "Red" or "Green" as the output.

For a Number field:
IIf(DateDiff("d",[Current Date],[Start])<8,1,2)

For a Text field:
IIf(DateDiff("d",[Current Date],[Start])<8,"Red","Green")

Hope that helps.