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Create date from number of days

I am trying to convert the number of days since Jan 01 1970 to JavaScript Date.
Here is the code snippet.
new Date(864e5 * parseInt(data[i].d));
//here data[i].d contains number of days.
I checked all the data by this.
console.log(typeof(data[i].d), data[i].d);
//prints
number 17674
but sometimes it unable to convert it into date.
Invalid Date {}
while for
number 17858
//outputs.
Fri Aug 17 2018 05:00:00 GMT+0500 (Pakistan Standard Time)
Thanks for your time.

You just have to add the number of days times the milliseconds in a day, like so:
var originalDay = new Date(864e5)
console.log(originalDay) //Thu Jan 01 1970 19:00:00 GMT-0500 (Eastern Standard Time)
var numOfDays = 7
var daysSince = new Date(864e5 + parseInt(numOfDays * 864e5))
console.log(daysSince) //Thu Jan 08 1970 19:00:00 GMT-0500 (Eastern Standard Time) --7 days later
To make this work for you, you would just have to replace that numOfDays with the values in your array.

Why .setUTCHours returns different results?

I have the following function which should read time in UTC and return in local time, and sometimes it adds 2 hours(correctly) and sometimes 4 hours. Why is that? What can be the reason?
value = time in UTC
toLocalDate(value) {
let string = new Date(value);
let date = new Date();
date.setUTCFullYear(string.getFullYear(), string.getMonth(), string.getDate());
date.setUTCHours(string.getHours(), string.getMinutes(), string.getSeconds(), 0);
return date;
}
Example data:
Value: 2017-08-23T06:00:00
Expected output: Wed Aug 23 2017 08:00:00 GMT+0200 (Central European Daylight Time)
Output: Wed Aug 23 2017 10:00:00 GMT+0200 (Central European Daylight Time)
On some devices(like my phone or computer) it returns the expected output.
On my friend's device(mobile) it returns the second output.

Why are those dates different?

Here the console output:
new Date(2016, 08, 22)
Thu Sep 22 2016 00:00:00 GMT+0200 (CEST)
new Date("2016, 08, 22")
Mon Aug 22 2016 00:00:00 GMT+0200 (CEST)
Different months but why ?

Javascript months are 0 based in the numeric case, but in the string parsing 08 is mapped to August as it is a string translation of "August" in standard date format.
Date is being invoked in different ways as mentioned here
new Date();
new Date(value);
new Date(dateString);
new Date(year,month[, day[, hour[, minutes[, seconds[, milliseconds]]]]]);
In your case, 3 & 4 formats are being called.
>> new Date(2016, 08, 22)
>> Thu Sep 22 2016 00:00:00 GMT+0530 (IST)
>> new Date(2016, 01, 22)
>> Mon Feb 22 2016 00:00:00 GMT+0530 (IST)
>> new Date(2016, 0, 22)
>> Fri Jan 22 2016 00:00:00 GMT+0530 (IST)
>> new Date("2016-08-22")
>> Mon Aug 22 2016 05:30:00 GMT+0530 (IST)
>> new Date("2016/08/22")
>> Mon Aug 22 2016 00:00:00 GMT+0530 (IST)
#RobG's input from the comments:
...parsing ofstrings other than ISO 8601 extended format is entirely
implementation dependent. The result of parsing "2016, 08, 22" could be anything, including an invalid Date.

The second Date constructor you use is intended to parse a (known) string representation of a date, like "Dec 25, 1995". The format you pass in is not a standard one, so even though the result is close to the correct date (and could be fixed by correcting the month value, as pointed out by DhruvPathak), it should not be used as results my differ depending on the runtime/browser.

Javascript vs Date

I have some interesting problem and can't find the solution. Look at this:
var d1 = new Date("07 31 2014");
document.write(d1);
document.write('<br />');
var d2 = new Date(1406746800 * 1000);
document.write(d2);
when I run this script I get this result:
Thu Jul 31 2014 00:00:00 GMT+0500 (UZT)
Thu Jul 31 2014 00:00:00 GMT+0500 (UZT)
then after I changed my time zone I get this result:
Thu Jul 31 2014 00:00:00 GMT-0800 (AKDT)
Wed Jul 30 2014 11:00:00 GMT-0800 (AKDT)
as you can see the second result is Jul 30, but first result is Jul 31. I think they must both be equal to 31 Jul. I know this problem depends on the timezone but is there a solution?

So the constructor parameter is:
an Integer value representing the number of milliseconds since 1
January 1970 00:00:00 UTC
So given your integer value, that represents (for me, in BST):
Wed Jul 30 2014 20:00:00 GMT+0100
Which is
Wed Jul 30 2014 19:00:00 UTC
And your timezone is GMT-8, so thats the above -8 which gives:
Wed Jul 30 2014 11:00:00 GMT-0800 AKDT
The date string constructor constructs the date in your local timezone. You can see what the value should be by doing this:
new Date("07 31 2014").getTime() / 1000
Which returns 1406761200, not 1406746800

Javascript UTC timestamp to Local Timezone

I'm trying to convert a timestamp being returned from a JSON resource in javascript that is displaying in UTC to the users local timezone. Below i'm trying to adjust with the user offset.
Example UTC output for date:
Tue Mar 27 2012 02:29:15 GMT-0400 (EDT)
Code
var date = new Date(data.date_created); //Data.date_created coming from json payload
var offset = date.getTimezoneOffset() //Get offset
var new_date = new Date(date offset); //Add offset to userdate
I'm struggling with the appropriate method to achieve this. Can anyone point me in the right direction?

I might be missing something but
var date = new Date( data.date_created );
does what I think you want.
>>> d=new Date('Tue Mar 27 2012 02:29:15 GMT-0800')
Date {Tue Mar 27 2012 06:29:15 GMT-0400 (EDT)}
>>> d.toLocaleString()
"Tue Mar 27 06:29:15 2012"
>>> d=new Date('Tue Mar 27 2012 02:29:15 GMT+0300')
Date {Mon Mar 26 2012 19:29:15 GMT-0400 (EDT)}
>>> d.toLocaleString()
"Mon Mar 26 19:29:15 2012"
Note how changing the GMT offset from -8 to +3 changes the resulting time by 11 hours.