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JS setHours on date is not working correctly

I have the following code to increment the hours in a date:
let timerExpireDate = new Date(countdownStartDate);
console.log(`original date is ${timerExpireDate}`);
console.log(`add on ${countdownHours} hours`);
timerExpireDate.setHours(timerExpireDate.getHours() + countdownHours);
console.log(`New date is ${timerExpireDate}`);
However it also seems to be incrementing the days by 6, here is the console log:
original date is Sun Jul 19 2020 16:36:39 GMT+0800 (Taipei Standard Time)
add on 2 hours
New date is Sat Jul 25 2020 18:36:39 GMT+0800 (Taipei Standard Time)
What am I doing wrong here?

It is likely that countdownHours is of type string instead of number, so timerExpireDate.getHours() + countdownHours is '162' (6 days later) instead of 18.
The fix is to cast countdownHours to number first, like countdownHours = +countdownHours.

Moment.js : Get first day of first week by given year

Im trying to get the first day of first week by given day
Correct results :
2017 = 2 Jan (Monday)
2016 = 4 Jan (Monday)
2015 = 1 Jan (Thursday)
2014 = 1 Jan (Wednesday)
2013 = 1 Jan (Tuesday)
I can get the first day of the year by :
moment().year(year).startOf('year') // All result is 1. jan XXXX
Then I tried : (correct)
moment().day(1).year(2017).week(1) // 2 Jan (Monday) (correct)
But when changed to 2016 : (wrong)
moment().day(1).year(2016).week(1) // 10 Jan (Sunday) (wrong)
Any know how to get correct results ? (Im open for pure JS date() also)
PS: Week number is based in Norway
Playground : https://jsfiddle.net/6sar7eb4/

There is no need to use loop, you can simply test if the week number is not 1 using week() that gives localized output
The output of moment#week will depend on the locale for that moment.
You have to set the locale to moment using locale() method to get start of week locale aware. If you have to show result in english you can change locale locally.
Here a working sample:
function getFirstDayOfFirstWeekByYear( year ) {
// Create moment object for the first day of the given year
let func = moment({year:year})
// Check if 1st of January is in the first week of the year
if( func.week() !== 1 ){
// If not, add a week to the first day of the current week
func.startOf('week').add(1, 'week');
}
// Return result using english locale
return func.locale('en').format('D. MMMM YYYY (dddd)')
}
// Set Norwegian Bokmål locale globally
moment.locale('nb');
// Tester
[2017, 2016, 2015, 2014, 2013].forEach(function ( year ) {
let s = '<li>' + getFirstDayOfFirstWeekByYear( year ) + '</li>'
document.getElementsByTagName('BODY')[0].innerHTML += s
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment-with-locales.min.js"></script>

Anyway if your happy to do it within a loop this works
function getFirstDayOfFirstWeekByYear( year ){
let D = 1;
x = 0;
while(x !== 1){
x = moment(`${year}-01-${D}`).isoWeek();
if(x === 1 ){
return moment(`${year}-01-${D}`).format('D. MMMM YYYY (dddd)')
}
D++;
};
}
results
2. January 2017 (Monday)
4. January 2016 (Monday)
1. January 2015 (Thursday)
1. January 2014 (Wednesday)
1. January 2013 (Tuesday)

Im trying to get the first day of first week by given day
Using moment.js to get the first day of the first week of the year based on the ISO 8601 scheme, you can just use the first day of the first week of the year since the ISO week starts on Monday:
// Return a moment.js object for the first weekday of the first week
// of the year using ISO 8601 scheme
function firstDay(year) {
return moment(year + '011', 'GGGGWWE');
}
// Examples
['2015', '2016', '2017', '2018', '2019', '2020']
.forEach(function(year) {
console.log('ISO First day of ' + year + ': ' +
firstDay(year).format('dddd D MMMM, YYYY'));
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
However you seem to not want to use the ISO scheme, so if you just want to get the first day of the year that isn't a Saturday or Sunday (i.e. ignore the ISO 8601 scheme), you can do (without moment.js):
// Return Date for first day of year that is one of
// Monday to Friday
function firstDay(year) {
var d = new Date(year, 0, 1);
var day = d.getDay();
d.setDate(d.getDate() + (day%6? 0 : (++day%4)));
return d;
}
// Examples
['2015', '2016', '2017', '2018', '2019', '2020']
.forEach(function(year) {
console.log('First weekday of ' + year + ': ' + firstDay(year).toString());
});
Discussion
There are many different schemes for first day of the week and first day of the year. A commonly used scheme is ISO 8601, where the first day of the week is Monday and the first week of the year is the one in which the first Thursday appears (or the first that contains at least 4 days of the week, it's the same result).
According to ISO 8601 rules, the first day of the first week of 2019 is Monday 31 December, 2018.
If you decide to support different schemes (e.g. in Saudi Arabia the week starts on Saturday and ends on Friday, with work days Saturday to Wednesday and in the US a week is typically Sunday to Saturday with work days Monday to Friday), then you should clearly state which scheme you support and not simply assume a scheme based on the language the user has chosen. Schemes may also be based on other cultural factors such as religion, so are not necessarily region or language-based.
Which scheme should be used for an English person in Saudi Arabia? An Islamic person in Bhutan? In Saudi the official language for commerce is French, but most international discussion is in English, while the national language is Arabic. But they use the Hijri calendar, which does not align with the Georgian calendar commonly used elsewhere and is not defined by language but by religion.
There may also be differences even in the same place, with departments of the same government choosing different schemes so even locale (in the correct meaning of the term) is not a definitive indicator of which scheme to use.
If you decide to support different schemes, far better to adopt a standard scheme (like ISO 8601) and let the user change it to the one they prefer if they wish (and which the preferences in my computer and calendar application allow me to do).

Why this datetime funtion setMonth result is wrong? getMonth() + 1

I have this code:
var nextDate = new Date("2016 01 31");
nextDate.setMonth(nextDate.getMonth() + 1);
I'm expecting the result to be Feb 28 2016, but it shows Mar 02 2016 instead.
Why? Is there any solution for it?

There is only 29 day in February, therefore, February 31 February will translate to Mars 2.
You need to update the days in your date object to the last day of that month. You can get the last day of the month by specifying a function that sets the date to 0:
function daysInMonth(month,year) {
return new Date(year, month, 0).getDate();
}

This is because February has 29 days, and when you set new month from January, which has 31 day, to February then the difference of the days are transferred to another month.
Easy way to do it is just create new Date instance.
You might need to implement some logic to get corresponding dates right

Possible work around with a helper function: after setMonth, check if the results doesn't contain a month equal to the expected month and if so, use setDate(0), which sets the date to last day of the previous month. e.g.
Date.prototype.addMonths = function(months){
var m = this.getMonth() + (months || 1);
this.setMonth(m);
if(this.getMonth() !== (m % 11)) //11: month is 0 based
this.setDate(0);
}
var nextDate = new Date("2016 01 31");
nextDate.addMonths(1);
document.writeln(nextDate);
months || 1 only is meant to have a default value if no month was submitted. m % 11 is needed in case of year transitions. 11 and not 12 because javascripts month (and thus getMonth) is 0 based.

Remove time from GMT time format

I am getting a date that comes in GMT format, Fri, 18 Oct 2013 11:38:23 GMT. The problem is that the time is messing up the timeline that I am using.
How can I strip out everything except for the actual date?

If you want to keep using Date and not String you could do this:
var d=new Date(); //your date object
console.log(new Date(d.setHours(0,0,0,0)));
-PS, you don't need a new Date object, it's just an example in case you want to log it to the console.
http://www.w3schools.com/jsref/jsref_sethours.asp

Like this:
var dateString = 'Mon Jan 12 00:00:00 GMT 2015';
dateString = new Date(dateString).toUTCString();
dateString = dateString.split(' ').slice(0, 4).join(' ');
console.log(dateString);

I'm using this workaround :
// d being your current date with wrong times
new Date(d.getFullYear(), d.getMonth(), d.getDate())

You could use Moment.js, a library that provides many helper functions to validate, manipulate, display and format dates and times in JavaScript.
Using Moment.js lib:
var dateString = new Date('Mon Jan 12 00:00:00 GMT 2015');
moment(dateString).format('YYYY-MM-DD HH:mm');
Or simplified:
moment('Mon Jan 12 00:00:00 GMT 2015').format('YYYY-MM-DD HH:mm')

Well,
Here is my Solution
let dateString = 'Mon May 25 01:07:00 GMT 2020';
let dateObj = new Date(dateString);
console.log(dateObj.toDateString());
// outputs Mon May 25 2020
See its documentation on MDN https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toDateString

Just cut it with substring:
var str = 'Fri, 18 Oct 2013 11:38:23 GMT';
str = str.substring(0,tomorrow.toLocaleString().indexOf(':')-3);

In this case you can just manipulate your string without the use of a Date object.
var dateTime = 'Fri, 18 Oct 2013 11:38:23 GMT',
date = dateTime.split(' ', 4).join(' ');
document.body.appendChild(document.createTextNode(date));

You can first convert the date to String:
String dateString = String.valueOf(date);
Then apply substring to the String:
dateString.substring(4, 11) + dateString.substring(30);
You need to take care as converting date to String will actually change the date format as well.

Javascript: Extract a date from a string?

I have a string formatted as either
Today 3:28AM
Yesterday 3:28AM
08/22/2011 3:28AM
What I need to do is somehow extract into a variable the date portion of my string, ie. 'Today', 'Yesterday' or a date formatted as DD/MM/YYYY.
Is something like this possible at all with Javascript?

Since the JavaScript date parser won't recognize your dates, you can write a parser that puts the date into a format that it will recognize. Here is a function that takes the date examples that you gave and formats them to get a valid date string:
function strToDate(dateStr) {
var dayTimeSplit = dateStr.split(" ");
var day = dayTimeSplit[0];
var time = dayTimeSplit[1];
if (day == "Today") {
day = new Date();
} else if (day == "Yesterday") {
day = new Date();
day.setDate(day.getDate() - 1);
} else {
day = new Date(day);
}
var hourMinutes = time.substring(0, time.length -2);
var amPM = time.substring(time.length -2, time.length);
return new Date((day.getMonth() + 1) + "/" + day.getDate() + "/" + day.getFullYear()
+ " " + hourMinutes + " " + amPM);
}
Then you can call stroToDate to convert your date formats to a valid JavaScript Date:
console.log(strToDate("Today 3:28AM"));
console.log(strToDate("Yesterday 3:28AM"));
console.log(strToDate("08/22/2011 3:28AM"));
Outputs:
Sun Sep 25 2011 03:28:00 GMT-0700 (Pacific Daylight Time)
Sat Sep 24 2011 03:28:00 GMT-0700 (Pacific Daylight Time)
Mon Aug 22 2011 03:28:00 GMT-0700 (Pacific Daylight Time)

Obviously "Today" and "Yesterday" can never be transformed back to a real numeric date, for now it seems that what are you trying to do here is to save it as "Today" and "Yesterday", right?
It appears that the dd/mm/yyyy hh:mmxx you specified is always separated by a space.
so you can just split the string into two, and save the first part as your date.
the javascript function:
http://www.w3schools.com/jsref/jsref_split.asp
As for how to transform from "Today" back to 26/09/2011 etc, you need to seek solution from the XML side.

Here is a similar question: Javascript equivalent of php's strtotime()?
Here is the linked article: http://w3schools.com/jS/js_obj_date.asp
And the suggested solution:
Basically, you can use the date constructor to parse a date
var d=new Date("October 13, 1975 11:13:00");

There are a couple of ways you could do this. I will offer 2 of them.
option1:
If the day always at the beginning of the string you could capture the the first part by using a regular expression like /([a-z0-9]*)\s|([0-9]{1,})\/([0-9]{1,})\/([0-9]{1,})\s/ <- im not the best regex writer.
option2:
You could also do a positive look ahead if the time come immediately after the day (like your example above. Here is a link with the proper syntax for JS regex. http://www.javascriptkit.com/javatutors/redev2.shtml you can scroll down to lookaheads and see an example that should get you suared away there.

var reTYD = /(today|yesterday|\d{1,2}\/\d{1,2}\/\d{4})/i;
console.log( myString.match(reTYD) );