Tracking Elapsed Time

 Tracking Elapsed Time

Problem

You want to track the elapsed time between events. 

Solution

Create a Date object when the first event occurs, a new Date object when the second event occurs, and subtract the first from the second. The difference is in milliseconds; to convert to seconds, divide by 1,000:

var firstDate = new Date();

setTimeout(function() {

 doEvent(firstDate);

}, 25000);

function doEvent() {

 var secondDate = new Date();

 var diff = secondDate - firstDate;

 console.log(diff); // approx. 25000

}

EXPLAIN

Some arithmetic operators can be used with Date, but with interesting results. In the

example, one Date instance can be subtracted from another, and the difference between

the two is returned as milliseconds. However, if you add two dates together, the result

is a string with the second Date instance concatenated to the first:

Thu Oct 08 2009 20:20:34 GMT-0500 (CST)Thu Oct 08 2009 20:20:31 GMT-0500 (CST)

If you divide the Date instances, the dates are converted to their millisecond value, and

the result of dividing one by the other is returned. Multiplying two dates will return a

very large millisecond result.