How to keep Night Shift™ enabled indefinitely on macOS 10.12.5+? Settings only have "turn on until tomorrow"
In the preferences (System Preferences > Display > Night Shift) do the following:


set 'Schedule' to Custom
set 'From:' to 00:00
set 'to:' to 23:59
adjust the Color Temperature as desired


Now night shift will be enabled all the time. Disable it using the Notification Center or using the 'Manual' toggle in the preferences pane.