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Adding "HH:MM" time format to Date Object Javascript properly

I have the requirement to set particular time of the day to Date Object. The Time is in String and is CET, so "16:00" means "15:00" in UTC in Winter time. The following code does the job in node.js on my local machine which is in CET Timezone:
addTimetoDate(new Date(),"16:00");
function addTimetoDate(theDate,theTime){
var dtDate = new Date(theDate)
try{
var strTime = theTime.replace(/ /g,'');
var hourArray = strTime.split(":");
dtDate.setHours(parseInt(hourArray[0]), parseInt(hourArray[1]), 0)
if (dtDate == "Invalid Date"){
dtDate = theDate;
}
} catch (e){
dtDate = theDate;
}
return dtDate
}
However when deployed to remote server it produces Date Object which is offset by one hour the other direction when displayed with toLocaleString it shows "17:00".
How to do it elegant way (as simple deduction of one hour will work only in Winter Time.
---EDIT---
To clarify the question is - Is there anything I can do prior to using .setHours to make it right. Or I should not use setHours but rather manipulate the string for Date Constructor, so 16.00 CET gets properly converted to UTC representation?

toLocaleString will convert the given time into the timezone defined by the locale,... and that's fine if the time is UTC to start with. But if it's already been offset, then it's going to offset it again, which is what you're seeing.
Time is a fiddly creature; when I'm working with time I always store it (eg in a database) as UTC and let the client descide how it gets displayed. That way I can guarantee no server-side silliness.

Ok Found the solution:
function addTimetoDate(theDate,theTime){
var d = new Date();
var hourLocal = d.toLocaleString({timezone:'Europe/Berlin'}).split(" ")[1].split(":")[0]
var hourISO = d.toISOString().split("T")[1].split(":")[0]
var diff = parseInt(hourLocal) - parseInt(hourISO);
var dtDate = new Date(theDate)
try{
var strTime = theTime.replace(/ /g,'');
var hourArray = strTime.split(":");
dtDate.setHours(parseInt(hourArray[0])-diff, parseInt(hourArray[1]), 0)
if (dtDate == "Invalid Date"){
dtDate = theDate;
}
} catch (e){
dtDate = theDate;
}
return dtDate
}
Diff in whatever situation - being on the server, or on the browser captures current status (including daylight saving change) of hourly difference for the given Timezone. Then you can adjust your given hour to UTC to use setHours.

Date calculation - Daylight saving time (DST) and Timezone issue - Turkey

Date calculation issue in JavaScript on Browser. There are 3 parameters -
From Date, No. of days & To Date
From Date selected using calendar component in JavaScript = 30/10/2016
No. of days entered = 2
Based on no. of days entered "To Date" should be calculated, so as per above input of From date & No. of days calculated "To Date" value should be 01/11/2016 but due to some wrong calculation it's showing 31/10/2016.
Time Zone - Istanbul, Turkey
Please refer below image for code snipped -
As it is clear from code snipped that prototype JavaScript library being used.
dateUtil.prototype.addDays=function(date,noofDays)
{
var _dateData=date.split("/");
var _date=eval(_dateData[0]);
var _month=eval(_dateData[1]);
var _year=eval(_dateData[2]);
var newFormatedDate = new Date(""+_month+"/"+_date+"/"+_year);
var newAddedDate=newFormatedDate.getTime() + noofDays*24*60*60*1000;
var theDate = new Date(newAddedDate);
var dd = theDate.getDate();
var mm = theDate.getMonth()+1; // 0 based
if(mm<10)
mm="0"+mm;
var yy = theDate.getYear();
if (yy < 1000)
yy +=1900; // Y2K fix
var addedDate=""+dd+"/"+mm+"/"+yy;
return addedDate;
}
It seems noofDays*24*60*60*1000 logic is problem where DST is not being considered.
There are 2 timezone showing with the same code but with different date format.
Please could you advise any guidance or read-up on this.
Edit :
JavaScript code added.

Probably not worth posting the code since it has some fundamental errors that should not have survived the new millennium.
var _date = eval(_dateDate[0]);
Don't use eval. There are a small number of cases where it is appropriate, but in general, just don't use it. Ever. The above is the same as:
var _date = _dateDate[0];
Then there is:
var newFormatedDate = new Date('' + _month + '/' + _date + '/' + _year)
You started on the right track by avoiding parsing strings with the Date constructor by splitting the date string into it's parts. But then you undid that good work by creating a new string and parsing it with Date. Just use parts directly:
var newFormatedDate = new Date(_year, _month-1, _date)
which removes all the vagaries of Date parsing and is less to type as well. Also, Date objects don't have a format, so a name like date is fine.
To add n days, just add them to the date:
var date = new Date(_year, _month-1, _date)
date.setDate(date.getDate() + 2);
So your function can be:
function dateUtil(){}
/* Add days to a date
** #param {string} date - date string in dd/mm/yyyy format
** #param {number} noofDays - number of days to add
** #returns {Date}
*/
dateUtil.prototype.addDays = function(date, noofDays) {
var dateData = date.split('/');
var date = new Date(dateData[2], dateData[1] - 1, dateData[0]);
date.setDate(date.getDate() + +noofDays);
return date;
}
var d = new dateUtil();
console.log(d.addDays('23/09/2016',3).toLocaleString());
I've use +noofDays to ensure it's a number. Also, the SO console seems to always write dates as ISO 8601 strings in Z time zone so I've used toLocaleString to keep it in the host time zone.

How to subtract two different dates from a date/time stamp?

I need to subtract a date like 1/26/2015 from a date-time like 2016-01-27T01:10:57.569000+00:00. From what I've read converting both to distance in milliseconds from Epoch and then subtracting is the easiest way. I've tried using various methods, but all the methods seem to say 2016-01-27T01:10:57.569000+00:00 is invalid data. The method .getTime() works great for the 1/26/2015 format, but it can't read the 2016-01-27T01:10:57.569000+00:00.
How does one go about getting the date/time UTC time into milliseconds?

On a complicated way you can use a regex to extract each part of the date as string and then use them in a new Date with all parameters:
function getTimeDifference(){
var regEx = /(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):([\d.]+)/;
var dateString = '2016-01-27T01:10:57.569000+00:00';
var r = regEx.exec( dateString );
var date1 = new Date(r[1], r[2]-1, r[3], r[4], r[5], r[6]); // Notice the -1 in the month
var date2 = new Date('1/26/2015');
var difference = date1 - date2;
Logger.log(difference);
}

I ended up using this. When I call parseDate(), I used getTime() to get the date in milliseconds then subtracted them and converted them to days. For my use case the time didn't have to be down to the second, but if it did, it wouldn't be hard to parse more info from the string. I ran into trouble initially because as a beginner Javascript writer I didn't know why apps script wouldn't accept this format into the date constructor.
function parseDate(str) {
//This should accept 'YYYY-MM-DD' OR '2016-01-27T01:10:57.569000+00:00'
if(str.length == 10){
var mdy = str.split('-');
return new Date(mdy[0], mdy[1]-1, mdy[2]);
}
else
{
var mdy = str.split('-');
var time = mdy[2].split('T');
var hms = time[1].split(':');
return new Date(mdy[0], mdy[1]-1, time[0], hms[0], hms [1]);
}
}

If you are confident that the values in the date strings will always be valid and that the ISO8601 string will always have offset 00:00 (i.e. UTC), then simple parse functions are:
// Parse ISO 8601 format 2016-01-27T01:10:57.569000+00:00
function parseISOUTC(s) {
var b = s.split(/\D/);
return new Date(Date.UTC(b[0],b[1]-1,b[2],b[3],b[4],b[5],b[6]));
}
document.write(parseISOUTC('2016-02-04T00:00:00.000+00:00'));
// Parse US format m/d/y
function parseMDY(s) {
var b = s.split(/\D/);
return new Date(b[2],b[0]-1,b[1]);
}
document.write('<br>'+ parseMDY('2/4/2016'))
document.write('<br>'+ (parseISOUTC('2016-02-04T00:00:00.000+00:00') - parseMDY('2/4/2016')))
Note that the first string is UTC and the second will be treated as local (per ECMAScript 2015), so the difference between 2016-02-04T00:00:00.000+00:00 and 2/4/2016 will be the time zone offset of the host system.

Changing date format javascript

I'm pulling some data from two different APIs and I want to the objects later on.
However, I'm getting two different date formats: this format "1427457730" and this format "2015-04-10T09:12:22Z". How can I change the format of one of these so I have the same format to work with?
$.each(object, function(index) {
date = object[index].updated_at;
}

Here's one option:
var timestamp = 1427457730;
var date = new Date(timestamp * 1000); // wants milliseconds, not seconds
var dateString = date.toISOString().replace(/\.\d+Z/, 'Z'); // remove the ms
dateString will now be 2015-03-27T12:02:10Z.

Try moment.js
var timestamp = 1427457730;
var date = '2015-04-10T09:12:22Z';
var m1 = moment(timestamp);
var m2 = moment(date);
console.log(m1);
console.log(m2);
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.11.1/moment.min.js"></script>
You can use .format() method in moment to parse the date to whatever format you want, just like:
m2.format('YYYY MMM DD ddd HH:mm:ss') // 2015 Apr 10 Fri 17:12:22
Check out the docs for more format tokens.

What you probably want in javascript, are date objects.
The first string is seconds since epoch, javascript needs milliseconds, so multiply it by 1000;
The second string is a valid ISO date, so if the string contains a hyphen just pass it into new Date.
var date = returned_date.indexOf('-') !== -1 ? returned_date : returned_date * 1000;
var date_object = new Date(date);
Making both types into date objects, you could even turn that into a handy function
function format_date(date) {
return new Date(date.indexOf('-') !== -1 ? date : date * 1000);
}
FIDDLE

Take a look at http://momentjs.com/. It is THE date/time formatting library for JavaScript - very simple to use, extremely flexible.

Create a Date object with CET timezone

To create Date object in UTC, we would write
new Date(Date.UTC(2012,02,30));
Without Date.UTC, it takes the locale and creates the Date object. If I have to create a Date object for CET running the program in some part of the world, how would I do it?

You don't create a JavaScript Date object "in" any specific timezone. JavaScript Date objects always work from a milliseconds-since-the-Epoch UTC value. They have methods that apply the local timezone offset and rules (getHours as opposed to getUTCHours), but only the local timezone. You can't set the timezone the Date object uses for its "local" methods.
What you're doing with Date.UTC (correctly, other than the leading 0 on 02) is just initializing the object with the appropriate milliseconds-since-the-Epoch value for that date/time (March 30th at midnight) in UTC, whereas new Date(2012, 2, 30) would have interpreted it as March 30th at midnight local time. There is no difference in the Date object other than the datetime it was initialized with.
If you need a timezone other than local, all you can do is use the UTC version of Date's functions and apply your own offset and rules for the timezone you want to use, which is non-trivial. (The offset is trivial; the rules tend not to be.)
If you go looking, you can find Node modules that handle timezones for you. A quick search for "node timezone" just now gave me timezone as the first hit. It also gave me links to this SO question, this SO question, and this list of timezone modules for Node.

function getCETorCESTDate() {
var localDate = new Date();
var utcOffset = localDate.getTimezoneOffset();
var cetOffset = utcOffset + 60;
var cestOffset = utcOffset + 120;
var cetOffsetInMilliseconds = cetOffset * 60 * 1000;
var cestOffsetInMilliseconds = cestOffset * 60 * 1000;
var cestDateStart = new Date();
var cestDateFinish = new Date();
var localDateTime = localDate.getTime();
var cestDateStartTime;
var cestDateFinishTime;
var result;
cestDateStart.setTime(Date.parse('29 March ' + localDate.getFullYear() + ' 02:00:00 GMT+0100'));
cestDateFinish.setTime(Date.parse('25 October ' + localDate.getFullYear() + ' 03:00:00 GMT+0200'));
cestDateStartTime = cestDateStart.getTime();
cestDateFinishTime = cestDateFinish.getTime();
if(localDateTime >= cestDateStartTime && localDateTime <= cestDateFinishTime) {
result = new Date(localDateTime + cestOffsetInMilliseconds);
} else {
result = new Date(localDateTime + cetOffsetInMilliseconds);
}
return result;
}