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Count Down Stoped After Changed the Date

I was trying to implement count down timer for my web site. I want to give two static dates to and get the count down running. I found an example that uses one hard code date and the other date is taken as new Date() . but when I change that new Date() to Hard Code values count down timer stopped . How to fix this issue .
Original Example I found in jsfiddle.net
My modified Example in jsfiddle.net
the only difference in those examples is i changed var date1 = new Date(); to var date1 = new Date("2017/07/22 20:30:00");

The dates that you provided will never change since you are continually calling the same function each interval (i.e. the difference between date1 and date2 will never change).
If you want a countdown, you'll need to use some relatively changing date similar to the original example you provided or retain an offset (i.e. store when you started the process and continually use an offset for your calculations) as seen below:
// Store a relative date to track passing time
var started = new Date();
showDiff();
function showDiff() {
// Keep track of the time that has elapsed
var offset = new Date() - started;
// Store your dates
var date1 = new Date("2017/07/22 20:30:00") - offset;
var date2 = new Date("2015/07/30 21:59:00");
// Calculate the differences
var diff = Math.abs(Math.floor((date2 - date1) / 1000));
var days = Math.floor(diff / (24 * 60 * 60));
var daysLeft = diff - days * 24 * 60 * 60;
var hours = Math.floor(daysLeft / (60 * 60));
var hoursLeft = daysLeft - hours * 60 * 60;
var minutes = Math.floor(hoursLeft / (60));
var minutesLeft = hoursLeft - minutes * 60;
var seconds = Math.floor(minutesLeft / 60);
var secondsLeft = minutesLeft - seconds * 60;
// Output
document.getElementById("showTime").innerHTML = "You have " + days + " days " + hours + " hours " + minutes + " minutes and " + secondsLeft + " seconds before death.";
setTimeout(showDiff, 1000);
}
<div id='showTime'></div>

Subtract the date from current date javascript [duplicate]

I'm creating an application which lets you define events with a time frame. I want to automatically fill in the end date when the user selects or changes the start date. I can't quite figure out, however, how to get the difference between the two times, and then how to create a new end Date using that difference.

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)

JavaScript perfectly supports date difference out of the box
https://jsfiddle.net/b9chris/v5twbe3h/
var msMinute = 60*1000,
msDay = 60*60*24*1000,
a = new Date(2012, 2, 12, 23, 59, 59),
b = new Date("2013 march 12");
console.log(Math.floor((b - a) / msDay) + ' full days between'); // 364
console.log(Math.floor(((b - a) % msDay) / msMinute) + ' full minutes between'); // 0
Now some pitfalls. Try this:
console.log(a - 10); // 1331614798990
console.log(a + 10); // mixed string
So if you have risk of adding a number and Date, convert Date to number directly.
console.log(a.getTime() - 10); // 1331614798990
console.log(a.getTime() + 10); // 1331614799010
My fist example demonstrates the power of Date object but it actually appears to be a time bomb

See JsFiddle DEMO
var date1 = new Date();
var date2 = new Date("2025/07/30 21:59:00");
//Customise date2 for your required future time
showDiff();
function showDiff(date1, date2){
var diff = (date2 - date1)/1000;
diff = Math.abs(Math.floor(diff));
var days = Math.floor(diff/(24*60*60));
var leftSec = diff - days * 24*60*60;
var hrs = Math.floor(leftSec/(60*60));
var leftSec = leftSec - hrs * 60*60;
var min = Math.floor(leftSec/(60));
var leftSec = leftSec - min * 60;
document.getElementById("showTime").innerHTML = "You have " + days + " days " + hrs + " hours " + min + " minutes and " + leftSec + " seconds before death.";
setTimeout(showDiff,1000);
}
for your HTML Code:
<div id="showTime"></div>

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));
}

Depending on your needs, this function will calculate the difference between the 2 days, and return a result in days decimal.
// This one returns a signed decimal. The sign indicates past or future.
this.getDateDiff = function(date1, date2) {
return (date1.getTime() - date2.getTime()) / (1000 * 60 * 60 * 24);
}
// This one always returns a positive decimal. (Suggested by Koen below)
this.getDateDiff = function(date1, date2) {
return Math.abs((date1.getTime() - date2.getTime()) / (1000 * 60 * 60 * 24));
}

If using moment.js, there is a simpler solution, which will give you the difference in days in one single line of code.
moment(endDate).diff(moment(beginDate), 'days');
Additional details can be found in the moment.js page
Cheers,
Miguel

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);
}

alternative modificitaion extended code..
http://jsfiddle.net/vvGPQ/48/
showDiff();
function showDiff(){
var date1 = new Date("2013/01/18 06:59:00");
var date2 = new Date();
//Customise date2 for your required future time
var diff = (date2 - date1)/1000;
var diff = Math.abs(Math.floor(diff));
var years = Math.floor(diff/(365*24*60*60));
var leftSec = diff - years * 365*24*60*60;
var month = Math.floor(leftSec/((365/12)*24*60*60));
var leftSec = leftSec - month * (365/12)*24*60*60;
var days = Math.floor(leftSec/(24*60*60));
var leftSec = leftSec - days * 24*60*60;
var hrs = Math.floor(leftSec/(60*60));
var leftSec = leftSec - hrs * 60*60;
var min = Math.floor(leftSec/(60));
var leftSec = leftSec - min * 60;
document.getElementById("showTime").innerHTML = "You have " + years + " years "+ month + " month " + days + " days " + hrs + " hours " + min + " minutes and " + leftSec + " seconds the life time has passed.";
setTimeout(showDiff,1000);
}

Below code will return the days left from today to futures date.
Dependencies: jQuery and MomentJs.
var getDaysLeft = function (date) {
var today = new Date();
var daysLeftInMilliSec = Math.abs(new Date(moment(today).format('YYYY-MM-DD')) - new Date(date));
var daysLeft = daysLeftInMilliSec / (1000 * 60 * 60 * 24);
return daysLeft;
};
getDaysLeft('YYYY-MM-DD');

<html>
<head>
<script>
function dayDiff()
{
var start = document.getElementById("datepicker").value;
var end= document.getElementById("date_picker").value;
var oneDay = 24*60*60*1000;
var firstDate = new Date(start);
var secondDate = new Date(end);
var diffDays = Math.round(Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay)));
document.getElementById("leave").value =diffDays ;
}
</script>
</head>
<body>
<input type="text" name="datepicker"value=""/>
<input type="text" name="date_picker" onclick="function dayDiff()" value=""/>
<input type="text" name="leave" value=""/>
</body>
</html>

this code fills the duration of study years when you input the start date and end date(qualify accured date) of study and check if the duration less than a year if yes the alert a message
take in mind there are three input elements the first txtFromQualifDate and second txtQualifDate and third txtStudyYears
it will show result of number of years with fraction
function getStudyYears()
{
if(document.getElementById('txtFromQualifDate').value != '' && document.getElementById('txtQualifDate').value != '')
{
var d1 = document.getElementById('txtFromQualifDate').value;
var d2 = document.getElementById('txtQualifDate').value;
var one_day=1000*60*60*24;
var x = d1.split("/");
var y = d2.split("/");
var date1=new Date(x[2],(x[1]-1),x[0]);
var date2=new Date(y[2],(y[1]-1),y[0])
var dDays = (date2.getTime()-date1.getTime())/one_day;
if(dDays < 365)
{
alert("the date between start study and graduate must not be less than a year !");
document.getElementById('txtQualifDate').value = "";
document.getElementById('txtStudyYears').value = "";
return ;
}
var dMonths = Math.ceil(dDays / 30);
var dYears = Math.floor(dMonths /12) + "." + dMonths % 12;
document.getElementById('txtStudyYears').value = dYears;
}
}

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.

Many answers here are based on a direct subtraction of Date objects like new Date(…) - new Date(…). This is syntactically wrong. Browsers still accept it because of backward compatibility. But modern JS linters will throw at you.
The right way to calculate date differences in milliseconds is new Date(…).getTime() - new Date(…).getTime():
// Time difference between two dates
let diffInMillis = new Date(…).getTime() - new Date(…).getTime()
If you want to calculate the time difference to now, you can just remove the argument from the first Date:
// Time difference between now and some date
let diffInMillis = new Date().getTime() - new Date(…).getTime()

function checkdate() {
var indate = new Date()
indate.setDate(dat)
indate.setMonth(mon - 1)
indate.setFullYear(year)
var one_day = 1000 * 60 * 60 * 24
var diff = Math.ceil((indate.getTime() - now.getTime()) / (one_day))
var str = diff + " days are remaining.."
document.getElementById('print').innerHTML = str.fontcolor('blue')
}

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);

how to get the days and exact minutes,exact hours from two dates in javascript?

I have start date time and end time,i need to split how many days , hours ,minutes in the two dates
for example ,
startdatetime = "09-06-2017 10:30"
enddatetime = "10-06-2017 11:45"
i need this result : 1 day 1 hour and 15 minutes
I try this one
var t = end - start;
var z = parseInt(t / 1000 / 60);
var time = display(z);
function display(a)
{
console.log(a);
var hours = Math.trunc(a/60);
var minutes = a % 60;
var one_day=1000*60*60*24
var days = Math.ceil(a/one_day)
var time = [hours,minutes,days];
return time;
}
i get the following 1day 24 hours and 15 minutes , can anyone help me , if its new logic means i will change into it,thanks in advance

Using momentjs, you can :
Parse your input string using moment(String, String)
Parse your input string using moment.utc
Get difference using diff() function
Create a duration from the difference value
Use duration days(), hours(), minutes() to get your result
Here a live sample:
var startdatetime = "2017-06-09T07:00:01.000Z";
var enddatetime = "2017-06-10T09:00:00.000Z";
// Parse input
var mStart = moment.utc(startdatetime);
var mEnd = moment.utc(enddatetime);
// Calculate difference and create duration
var dur = moment.duration( mEnd.diff(mStart) );
// Show the result
console.log(dur.days() + ' days ' + dur.hours() + ' hour ' + dur.minutes() + ' minutes');
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
If you want you can use moment-duration-format plug-in to get the same result using format() method on duration. Here a working sample:
var startdatetime = "2017-06-09T07:00:01.000Z";
var enddatetime = "2017-06-10T09:00:00.000Z";
// Parse input
var mStart = moment.utc(startdatetime);
var mEnd = moment.utc(enddatetime);
// Calculate difference and create duration
var dur = moment.duration( mEnd.diff(mStart) );
// Show the result
console.log(dur.format('d [day] h [hour] m [minutes]'));
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment-duration-format/1.3.0/moment-duration-format.min.js"></script>

Well, if you look at documentation for javascript Date objects, there is a getTime() method . You can also use the valueOf() method. They both return the number of milliseconds representing your Date object.
You can simply call that on both Date objects and then find the difference. Once you have the difference you can find the amount of secs, mins , hrs, days, etc. Here is an example:
var start = new Date(*some date*);
var end = new Date(*some date*);
var dif = end.valueOf() - start.valueOf();
if (dif >= 0) {
var secs = Math.floor(dif / 1000 % 60);
var mins = Math.floor(dif / 1000 / 60 % 60);
var hrs = Math.floor(dif / 1000 / 60 / 60 % 24);
var days =
Math.floor(dif / 1000 / 60 / 60 / 24 % 365);
var yrs =
Math.floor(dif / 1000 / 60 / 60 / 24 / 365);

Try the following:
var t = end - start;
var z = parseInt(t / 1000 / 60);
var time = display(z);
function display(minutes)
{
var hours = (minutes / 60 | 0) % 24;
var minutes = (minutes | 0) % 60;
var days = minutes / 60 / 24 | 0;
return [hours, minutes, days];
}
Note that in javascript, doing x | 0 is the same as Math.floor(x).

It looks to me like your calculation for hours still has the days in it. Once you have established the days, just subtract those out when you calculate the hours.
var start = new Date("June 09, 2017 10:30:00");
var end = new Date("June 10, 2017 11:45:00");
var t = end - start;
var z = parseInt(t / 1000 / 60);
var time = display(z);
console.log(time);
function display(a)
{
var minutes = a % 60;
var one_day=1000*60*60*24
var days = Math.ceil(a/one_day)
var hours = Math.trunc((a-(days*1440))/60);
var time = [hours,minutes,days];
return time;
}
Having said that, I highly recommend moment.js to handle this type of thing, if you can.

var startDateTime = 1497029400000;
var endDateTime = 1497120300000;
var timeDifference = endDateTime - startDateTime
// with the given dates, days equals 1.0520833333333333
// we want to extract the trailing decimal values using modulus to get the other times
function getTimeDifference(timeDifference) {
var days = timeDifference/1000/60/60/24
days >= 1
? var dayCount = Math.trunc(days); // store the day count
: var dayCount = 0; // it is less than one day
// get the remaining hours
var hours = (days % 1) * 24;
var hoursCount = Math.trunc((days % 1) * 24);
// get the remaining minutes
var minutesCount = Math.ceil((hours % 1) * 60);
}

Comparing two Dates and returning in correct format

I have a requirement where I need to compare two date values (Past Date and the Present Date) and return the exact difference in year/month/day and if it exceeds 59 years 6 month even a single day then it should return over age else under age
I searched in web but not getting anything relevant to it.
Currently I am trying:
var start_date = new Date("1990-04-25");
var end_date = new Date();
var total_months = (end_date.getFullYear() - start_date.getFullYear())*12 + (end_date.getMonth() - start_date.getMonth());
if (total_months < 714) {
alert("under age")
} else { alert("over age") }
where I am converting the years to month, which is not giving me the exact result.
Can some one please help me to solve this.

var start_date = new Date("1990-04-25");
var end_date = new Date();
var millisec = Math.abs(end_date - start_date); // it will provide you miliseconds
// here you can convert it to days or month and check for over or under age
e.g
var seconds = (millisec / 1000).toFixed(1);
var minutes = (millisec / (1000 * 60)).toFixed(1);
var hours = (millisec / (1000 * 60 * 60)).toFixed(1);
var days = (millisec / (1000 * 60 * 60 * 24)).toFixed(1);
for More help refer to these :
1. How to subtract date/time in javascript?
2. Convert time interval given in seconds into more human readable form

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

I'm creating an application which lets you define events with a time frame. I want to automatically fill in the end date when the user selects or changes the start date. I can't quite figure out, however, how to get the difference between the two times, and then how to create a new end Date using that difference.

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)

JavaScript perfectly supports date difference out of the box
https://jsfiddle.net/b9chris/v5twbe3h/
var msMinute = 60*1000,
msDay = 60*60*24*1000,
a = new Date(2012, 2, 12, 23, 59, 59),
b = new Date("2013 march 12");
console.log(Math.floor((b - a) / msDay) + ' full days between'); // 364
console.log(Math.floor(((b - a) % msDay) / msMinute) + ' full minutes between'); // 0
Now some pitfalls. Try this:
console.log(a - 10); // 1331614798990
console.log(a + 10); // mixed string
So if you have risk of adding a number and Date, convert Date to number directly.
console.log(a.getTime() - 10); // 1331614798990
console.log(a.getTime() + 10); // 1331614799010
My fist example demonstrates the power of Date object but it actually appears to be a time bomb

See JsFiddle DEMO
var date1 = new Date();
var date2 = new Date("2025/07/30 21:59:00");
//Customise date2 for your required future time
showDiff();
function showDiff(date1, date2){
var diff = (date2 - date1)/1000;
diff = Math.abs(Math.floor(diff));
var days = Math.floor(diff/(24*60*60));
var leftSec = diff - days * 24*60*60;
var hrs = Math.floor(leftSec/(60*60));
var leftSec = leftSec - hrs * 60*60;
var min = Math.floor(leftSec/(60));
var leftSec = leftSec - min * 60;
document.getElementById("showTime").innerHTML = "You have " + days + " days " + hrs + " hours " + min + " minutes and " + leftSec + " seconds before death.";
setTimeout(showDiff,1000);
}
for your HTML Code:
<div id="showTime"></div>

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));
}

Depending on your needs, this function will calculate the difference between the 2 days, and return a result in days decimal.
// This one returns a signed decimal. The sign indicates past or future.
this.getDateDiff = function(date1, date2) {
return (date1.getTime() - date2.getTime()) / (1000 * 60 * 60 * 24);
}
// This one always returns a positive decimal. (Suggested by Koen below)
this.getDateDiff = function(date1, date2) {
return Math.abs((date1.getTime() - date2.getTime()) / (1000 * 60 * 60 * 24));
}

If using moment.js, there is a simpler solution, which will give you the difference in days in one single line of code.
moment(endDate).diff(moment(beginDate), 'days');
Additional details can be found in the moment.js page
Cheers,
Miguel

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);
}

alternative modificitaion extended code..
http://jsfiddle.net/vvGPQ/48/
showDiff();
function showDiff(){
var date1 = new Date("2013/01/18 06:59:00");
var date2 = new Date();
//Customise date2 for your required future time
var diff = (date2 - date1)/1000;
var diff = Math.abs(Math.floor(diff));
var years = Math.floor(diff/(365*24*60*60));
var leftSec = diff - years * 365*24*60*60;
var month = Math.floor(leftSec/((365/12)*24*60*60));
var leftSec = leftSec - month * (365/12)*24*60*60;
var days = Math.floor(leftSec/(24*60*60));
var leftSec = leftSec - days * 24*60*60;
var hrs = Math.floor(leftSec/(60*60));
var leftSec = leftSec - hrs * 60*60;
var min = Math.floor(leftSec/(60));
var leftSec = leftSec - min * 60;
document.getElementById("showTime").innerHTML = "You have " + years + " years "+ month + " month " + days + " days " + hrs + " hours " + min + " minutes and " + leftSec + " seconds the life time has passed.";
setTimeout(showDiff,1000);
}

Below code will return the days left from today to futures date.
Dependencies: jQuery and MomentJs.
var getDaysLeft = function (date) {
var today = new Date();
var daysLeftInMilliSec = Math.abs(new Date(moment(today).format('YYYY-MM-DD')) - new Date(date));
var daysLeft = daysLeftInMilliSec / (1000 * 60 * 60 * 24);
return daysLeft;
};
getDaysLeft('YYYY-MM-DD');

<html>
<head>
<script>
function dayDiff()
{
var start = document.getElementById("datepicker").value;
var end= document.getElementById("date_picker").value;
var oneDay = 24*60*60*1000;
var firstDate = new Date(start);
var secondDate = new Date(end);
var diffDays = Math.round(Math.abs((firstDate.getTime() - secondDate.getTime())/(oneDay)));
document.getElementById("leave").value =diffDays ;
}
</script>
</head>
<body>
<input type="text" name="datepicker"value=""/>
<input type="text" name="date_picker" onclick="function dayDiff()" value=""/>
<input type="text" name="leave" value=""/>
</body>
</html>

this code fills the duration of study years when you input the start date and end date(qualify accured date) of study and check if the duration less than a year if yes the alert a message
take in mind there are three input elements the first txtFromQualifDate and second txtQualifDate and third txtStudyYears
it will show result of number of years with fraction
function getStudyYears()
{
if(document.getElementById('txtFromQualifDate').value != '' && document.getElementById('txtQualifDate').value != '')
{
var d1 = document.getElementById('txtFromQualifDate').value;
var d2 = document.getElementById('txtQualifDate').value;
var one_day=1000*60*60*24;
var x = d1.split("/");
var y = d2.split("/");
var date1=new Date(x[2],(x[1]-1),x[0]);
var date2=new Date(y[2],(y[1]-1),y[0])
var dDays = (date2.getTime()-date1.getTime())/one_day;
if(dDays < 365)
{
alert("the date between start study and graduate must not be less than a year !");
document.getElementById('txtQualifDate').value = "";
document.getElementById('txtStudyYears').value = "";
return ;
}
var dMonths = Math.ceil(dDays / 30);
var dYears = Math.floor(dMonths /12) + "." + dMonths % 12;
document.getElementById('txtStudyYears').value = dYears;
}
}

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.

Many answers here are based on a direct subtraction of Date objects like new Date(…) - new Date(…). This is syntactically wrong. Browsers still accept it because of backward compatibility. But modern JS linters will throw at you.
The right way to calculate date differences in milliseconds is new Date(…).getTime() - new Date(…).getTime():
// Time difference between two dates
let diffInMillis = new Date(…).getTime() - new Date(…).getTime()
If you want to calculate the time difference to now, you can just remove the argument from the first Date:
// Time difference between now and some date
let diffInMillis = new Date().getTime() - new Date(…).getTime()

function checkdate() {
var indate = new Date()
indate.setDate(dat)
indate.setMonth(mon - 1)
indate.setFullYear(year)
var one_day = 1000 * 60 * 60 * 24
var diff = Math.ceil((indate.getTime() - now.getTime()) / (one_day))
var str = diff + " days are remaining.."
document.getElementById('print').innerHTML = str.fontcolor('blue')
}

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);