Horologe provides a digital clock widget for the macOS Notification Center because the built in Clock app only supports analog clocks.
Download the latest release, uncompress, drag Horologe.app to /Applications. Then, launch Horologe.app.
You may now quit the application.
NOTE: You must open Horologe.app at least once for the widget to become available.
Next, open Notification Center and click the "Edit Widgets" button all the way at the bottom of the list. Select the Horologe widget
You have successfully installed the widget.