Please add the excel function networkdays to SharePoint calculated columns, this should be a standard feature! And I believe is the biggest problem especially with SharePoint online office 365 which has been locked down from the solutions capability to sandbox event receiver solutions code.

Many business use SharePoint to host there business data and struggle to do datediff Calc a for workflows and calculated columns, why isn't this standard feature?

https://support.office.com/en-us/article/NETWORKDAYS-function-48e717bf-a7a3-495f-969e-5005e3eb18e7