The Date documentation states that :
The JavaScript date is based on a time value that is milliseconds since midnight January 1, 1970, UTC
Click on start button then on end button. It will show you the number of seconds between the 2 clicks.
The milliseconds diff is in variable timeDiff. Play with it to find seconds/minutes/hours/ or what you need
var startTime, endTime;_x000D_
_x000D_
function start() {_x000D_
startTime = new Date();_x000D_
};_x000D_
_x000D_
function end() {_x000D_
endTime = new Date();_x000D_
var timeDiff = endTime - startTime; //in ms_x000D_
// strip the ms_x000D_
timeDiff /= 1000;_x000D_
_x000D_
// get seconds _x000D_
var seconds = Math.round(timeDiff);_x000D_
console.log(seconds + " seconds");_x000D_
}<button onclick="start()">Start</button>_x000D_
_x000D_
<button onclick="end()">End</button>OR another way of doing it for modern browser
Using performance.now() which returns a value representing the time elapsed since the time origin. This value is a double with microseconds in the fractional.
The time origin is a standard time which is considered to be the beginning of the current document's lifetime.
var startTime, endTime;_x000D_
_x000D_
function start() {_x000D_
startTime = performance.now();_x000D_
};_x000D_
_x000D_
function end() {_x000D_
endTime = performance.now();_x000D_
var timeDiff = endTime - startTime; //in ms _x000D_
// strip the ms _x000D_
timeDiff /= 1000; _x000D_
_x000D_
// get seconds _x000D_
var seconds = Math.round(timeDiff);_x000D_
console.log(seconds + " seconds");_x000D_
}<button onclick="start()">Start</button>_x000D_
<button onclick="end()">End</button>