What's an easy to use timer for studying and staying focused?

https://pomofocus.io/

The Pomodoro Technique is a time management method developed by Francesco Cirillo in the late 1980s. The technique uses a timer to break down work into intervals, traditionally 25 minutes in length, 
A Pomodoro is 25 minutes because it's the perfect length of time to help you not procrastinate, be productive, create urgency and help you stay focused on any task you are tackling.