This datapack adds in a stopwatch which can be toggled on and off. The time is shown in the right of the screen in seconds

/function sw:start starts the stopwatch
/function sw:stop stops the stopwatch
/function sw:reset resets the stopwatch

Step one: Download the datapack
Step two: Extract the file
Step three: Drag it into your datapacks folder of the world you want to implement it into
Step four: Enter the world and type /reload. If a message appears it has worked successfully
Step five: If you do not have cheats enabled click open to lan >> cheats on, then type /reload  

Works from 1.13 to 1.16
Multiplayer not supported