How to set-up an on-screen timer First, open up Notebook on the computer you're presenting on and then select the gallery item (2nd tab down in the menu shown on the very left of the Notebook window). The gallery browser will then open enabling you to type "timer" into the search bar near the top of the gallery browser pane:
The results of the search will be displayed in the lower half of the gallery pane, and if you click the little arrow within the section labelled 'Interactive and Multimedia' you'll reveal the 3 results. Simply drag the blue, circular timer onto the slide and then resize if necessary, as shown. To make the timer appear on top of your PowerPoint slideshow click on the view icon and then choose the 'Transparent Background' option (note that - depending on the version of Notebook you're using - you may instead see the different view icons/options displayed separately within Notebook's menu bar):
The Notebook software will then disappear leaving the timer visible on screen (along with some Notebook controls). You can then launch your PowerPoint and the timer should remain on top (and can be repositioned via clicking on it's frame and dragging to a new location) of any content beneath.