Started using OpenProject (trial) today and tried to add a column "Duration" for a particular task.   I was surprised to see that this is not in the drop down as an option.  E.g. if you have a task starting 28/09/2020 and finishing 27/10/2020 it would be useful to add "Duration" showing the number of working days between these two dates.

Good to knpw for other users: This is the feature request regarding this topic: #34703