var secondBetweenTwoDate = Math.abs((new Date().getTime() - oldDate.getTime()) / 1000);
// date1 and date2 are date objects
let hours = Math.abs(date1 - date2) / 36e5;
// The subtraction returns the difference between the two dates in milliseconds.
// 36e5 is the scientific notation for 60*60*1000, dividing by which converts
// the milliseconds difference into hours.
var diff = Math.abs(new Date('2011/10/09 12:00') - new Date('2011/10/09 00:00'));