how to work out how many seconds has passed since a date react code example

Example 1: javascript get seconds between two dates

var secondBetweenTwoDate = Math.abs((new Date().getTime() - oldDate.getTime()) / 1000);

Example 2: js date difference in seconds

var t1 = new Date();
var t2 = new Date();
var dif = t1.getTime() - t2.getTime();