How do I get the difference between two Dates in JavaScript?

In JavaScript, dates can be transformed to the number of milliseconds since the epoc by calling the getTime() method or just using the date in a numeric expression.

So to get the difference, just subtract the two dates.

To create a new date based on the difference, just pass the number of milliseconds in the constructor.

var oldBegin = ...
var oldEnd = ...
var newBegin = ...

var newEnd = new Date(newBegin + oldEnd - oldBegin);

This should just work

EDIT: Fixed bug pointed by @bdukes

EDIT:

For an explanation of the behavior, oldBegin, oldEnd, and newBegin are Date instances. Calling operators + and - will trigger Javascript auto casting and will automatically call the valueOf() prototype method of those objects. It happens that the valueOf() method is implemented in the Date object as a call to getTime().

So basically: date.getTime() === date.valueOf() === (0 + date) === (+date)

If you don't care about the time component, you can use .getDate() and .setDate() to just set the date part.

So to set your end date to 2 weeks after your start date, do something like this:

function GetEndDate(startDate)
{
    var endDate = new Date(startDate.getTime());
    endDate.setDate(endDate.getDate()+14);
    return endDate;
}

To return the difference (in days) between two dates, do this:

function GetDateDiff(startDate, endDate)
{
    return endDate.getDate() - startDate.getDate();
}

Finally, let's modify the first function so it can take the value returned by 2nd as a parameter:

function GetEndDate(startDate, days)
{
    var endDate = new Date(startDate.getTime());
    endDate.setDate(endDate.getDate() + days);
    return endDate;
}

Thanks @Vincent Robert, I ended up using your basic example, though it's actually newBegin + oldEnd - oldBegin. Here's the simplified end solution:

    // don't update end date if there's already an end date but not an old start date
    if (!oldEnd || oldBegin) {
        var selectedDateSpan = 1800000; // 30 minutes
        if (oldEnd) {
            selectedDateSpan = oldEnd - oldBegin;
        }

       newEnd = new Date(newBegin.getTime() + selectedDateSpan));
    }

function compare()
{
  var end_actual_time    = $('#date3').val();

  start_actual_time = new Date();
  end_actual_time = new Date(end_actual_time);

  var diff = end_actual_time-start_actual_time;

  var diffSeconds = diff/1000;
  var HH = Math.floor(diffSeconds/3600);
  var MM = Math.floor(diffSeconds%3600)/60;

  var formatted = ((HH < 10)?("0" + HH):HH) + ":" + ((MM < 10)?("0" + MM):MM)
  getTime(diffSeconds);
}
function getTime(seconds) {
  var days = Math.floor(leftover / 86400);

  //how many seconds are left
  leftover = leftover - (days * 86400);

  //how many full hours fits in the amount of leftover seconds
  var hours = Math.floor(leftover / 3600);

  //how many seconds are left
  leftover = leftover - (hours * 3600);

  //how many minutes fits in the amount of leftover seconds
  var minutes = leftover / 60;

  //how many seconds are left
  //leftover = leftover - (minutes * 60);
  alert(days + ':' + hours + ':' + minutes);
}

If you use Date objects and then use the getTime() function for both dates it will give you their respective times since Jan 1, 1970 in a number value. You can then get the difference between these numbers.

If that doesn't help you out, check out the complete documentation: http://www.w3schools.com/jsref/jsref_obj_date.asp

var getDaysLeft = function (date1, date2) {
   var daysDiffInMilliSec = Math.abs(new Date(date1) - new Date(date2));
   var daysLeft = daysDiffInMilliSec / (1000 * 60 * 60 * 24);   
   return daysLeft;
};
var date1='2018-05-18';
var date2='2018-05-25';
var dateDiff = getDaysLeft(date1, date2);
console.log(dateDiff);

To get the date difference in milliseconds between two dates:

var diff = Math.abs(date1 - date2);

I'm not sure what you mean by converting the difference back into a date though.

THIS IS WHAT I DID ON MY SYSTEM.

var startTime=("08:00:00").split(":");
var endTime=("16:00:00").split(":");
var HoursInMinutes=((parseInt(endTime[0])*60)+parseInt(endTime[1]))-((parseInt(startTime[0])*60)+parseInt(startTime[1]));
console.log(HoursInMinutes/60);