Getting the difference between 2 dates in Javascript in hours, minutes, seconds with UTC

Question

Like the title says, I need to get the difference between two dates and display the hours, minutes, seconds that counts down to the finish date. I have this:

The instructions for my assignment state that I need to: "Choose an end date in the future (several weeks after assignment is due) and use UTC. Display the hours, minutes, and seconds left until the auction ends." I'm kind of lost on how to implement the UTC and create a proper countdown? I'm not even sure if I did any of it right (I am just starting to learn j.s). How should I fix my code?

Answer 1

In your code you have:

var date1 = new Date().getTime();

There's no need to use getTime in this case. But it is helpful to use meaningful variable names:

var now = new Date();

Then there's:

var date2 = new Date("05/29/2017").getTime();

Don't use the Date constructor (or Date.parse) to parse strings as it's mostly implementation dependent and inconsistent. If you have a specific date, pass values directly to the constructor.

Regarding "use UTC", that's a bit confusing as ECMAScript Date objects are UTC. They use the host timezone offset to calculate an internal UTC time value, and also to display "local" date and time values. The only way I can interpret "use UTC" is to use Date.UTC to create a date instance, e.g.:

var endDate = new Date(Date.UTC(2017,4,29)); // 2017-05-29T00:00:00Z

Now you can get the difference between then an now using:

var diff = endDate - now; 

When trying to get the hours, minutes and seconds you have:

var seconds = diff / 1000;
var minutes = (diff / 1000) / 60;
var hours = minutes / 60;

That converts the entire difference to each of hours, minutes and seconds so the total time is about 3 times what it should be. What you need is just the components, so:

var hours = Math.floor(diff / 3.6e5);
var minutes = Math.floor(diff % 3.6e5) / 6e4);
var seconds = Math.floor(diff % 6e4) / 1000;

Putting it all together in one function:

function timeLeft() {
    var now = new Date();
    var endDate = new Date(Date.UTC(2017,4,29)); // 2017-05-29T00:00:00Z
    var diff = endDate - now; 

    var hours   = Math.floor(diff / 3.6e6);
    var minutes = Math.floor((diff % 3.6e6) / 6e4);
    var seconds = Math.floor((diff % 6e4) / 1000);
    console.log('Time remaining to ' + endDate.toISOString() + 
                ' or\n' + endDate.toString() + ' local is\n' +
                 hours + ' hours, ' +  minutes + ' minutes and ' + 
                 seconds + ' seconds');
}

timeLeft()

There are many, many questions and answers here about timers and date differences, do some searches.