The old user interface looked something like this:
The updated app replaces the top navigation with NavigationView control on the left hand side. Also, Timer section is now the home screen of the app. Microsoft has also redesigned Timer, Alarm, World Clock and Stopwatch sections to offer better user experience. Throughout the new app experience, you can find new animations, floating buttons, rounded corners and more.
To get the new look, check for updates in the Microsoft Store.