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convert date object to datestring in TZ format javascript

My input is
var dt = "06/01/2018"
var time = "06:25:00"
i want output to be in string like this "2018-06-01T00:55:00.000Z".
I did var result = new Date(dt+time); //output is object here
I want to convert that object to string. Can any one tell me how to do that.

There is no need to use a Date object if the dt and time formats are known in advance. Here is how you can do it
const dt = '06/01/2018';
const [mm,dd,yyyy] = dt.split('/')
const time = '06:25:00';
const date = `${yyyy}-${mm}-${dd}T${time}.000Z`;
console.log(date);

const dt = '06/01/2018';
var time = "06:25:00";
console.log(new Date(`${dt} ${time}`).toJSON())

var dt = "06/01/2018";
var time = "06:25:00";
var date_test = new Date((dt + ' ' + time)).toString();
console.log(date_test);

Convert UTC to local time Javascript

I'm trying to convert UTC time to local time, but the below code is not working. What's wrong in it?
var parsedStartDateTime =
new Date(moment.unix(parseInt(data['StartDateTime'].substr(6)) / 1000));
var startDateTimeMoment =
moment.tz(parsedStartDateTime, tzName);
var formatted_date =
startDateTimeMoment.format("MMM DD YYYY h:mm:ss A");

To format your date try this:
var d = new Date();
var formatD = d.toLocaleFormat("%d.%m.%Y %H:%M (%a)");
Reference: Javascript to convert UTC to local time

Try appending UTC to the string before converting it to a date then use toString() method of date.
Example:
var myDate = new Date('7/1/2014 5:22:55 PM UTC');
date.toString(); //this should give you local date and time
This code was taken from here

Here is my solution:
function convertUTCDateToLocalDate(date) {
var newDate = new Date(date.getTime()+date.getTimezoneOffset()*60*1000);
var offset = date.getTimezoneOffset() / 60;
var hours = date.getHours();
newDate.setHours(hours - offset);
return newDate;
}
var date = convertUTCDateToLocalDate(new Date(date_string_you_received));
date.toLocaleString().replace(/GMT.*/g,"");

Time from X not working as expected moment.js

$.each(data[i].replies, function(m, n) {
var currentdate = new Date();
console.log(n.entry.date_entered);
check = moment(n.entry.date_entered, 'YYYY/MM/DD');
check1 = moment(currentdate, 'YYYY/MM/DD');
console.log(check);
console.log(check1);
var month = check.format('M');
var day = check.format('DD');
var year = check.format('YYYY');
var month1 = check1.format('M');
var day1 = check1.format('DD');
var year1 = check1.format('YYYY');
get = moment([year, month, day]);
get1 = moment([year1, month1, day1]);
g = get1.from(get);
});
Sample n.entry.date_entered : 2014-07-28 12:23:43
For all the dates i am getting a few seconds ago don't know why

I think your problem is the format mask that you pass in to moment.
In your sample you use - as the delimiter but in your format mask you use /. This way moment will not be able to parse the date and will give you the current date instead.
Try changing your format mask to "YYYY-MM-DD".

calculate difference between two timestamp and get difference in unix timestamp

I want to calculate the difference between two dateTime, one date is submitted by user and other is current time:
user submitted time - now = difference in unix
user submitted time format is:
2014-03-26 10:52:00
Thanks for your help.

You can simply do this with getTime() which returns the number of milliseconds.
var ds = "2014-03-26 10:52:00";
var newDate = new Date(ds).getTime(); //convert string date to Date object
var currentDate = new Date().getTime();
var diff = currentDate-newDate;
console.log(diff);
Sometimes there are chance for cross browser compatibility in parsing the date string so it is better to parse it like
var ds = "2014-03-26 10:52:00";
var dateArray = ds.split(" "); // split the date and time
var ds1 = dateArray[0].split("-"); // split each parts in date
var ds2 = dateArray[1].split(":"); // split each parts in time
var newDate = new Date(ds1[0], (+ds1[1] - 1), ds1[2], ds2[0], ds2[1], ds2[2]).getTime(); //parse it
var currentDate = new Date().getTime();
var diff = currentDate - newDate;
console.log(diff); //timestamp difference

You can use MomentJS library
var user_submited_time = moment('2014-03-26 10:52:00');
var now = moment();
var value = user_submited_time - now;

Calculate difference between two calendar dates in javascript

I need to calculate difference between two calendar dates. I have gone through various posts but the value returned is not correct.
Heres' my code:-
function getTotalDays()
{
var date11 = document.getElementById("departure_date").value;
var date22 = document.getElementById("arrival_date").value;
var one_day=1000*60*60*24;
var date1 = new Date(date11);
var date2 = new Date(date22);
// Convert both dates to milliseconds
var date1_ms = date1.getTime();
var date2_ms = date2.getTime();
// Calculate the difference in milliseconds
var difference_ms = date2_ms - date1_ms;
// Convert back to days and return
var diffDays = Math.round(difference_ms/one_day);
alert(diffDays);
}
suppse the difference is 2 days its showing as 59.
What's wrong..??

The values you are passing to the date object are likely wrong. Its probably easier for you to do something like this:
var date1 = getDate(date11);
var date2 = getDate(date22);
with getDate being:
function getDate(date) {
//date format dd/mm/yyyy
var dateArr = date.split('/');
var date = new Date(dateArr[2], dateArr[1], dateArr[0]);
return date;
}