Merlin provides a user friendly interface to modifying the project calendar.
Project calendar can be accessed in a tab of the inspector when the top most row (row “#0”) in the activities view is selected.
In the “Regular” tab you may modify the work intervals of the working week days if required.
You may also choose to change the preselected project calendar all together and switch from “Standard” to “Night Shift” for example.
Just select the desired project calendar from the appropriate pop up on the top right area of the inspector and make it your main project calendar afterward.
Please find hereafter a screencast of those actions