Develop Reference

Get today's day from Date.now()

After getting the value from Date.now() I have tried extracting the date like that:
Date.now().getDate()
However, the getDate() function doesn't work on it. My whole application is based on dates that are retrieved from Date.now() and I need to get current day of the month from the Date.now() value. I have done some research and I can not find a solution to it.

Date.now() doesn't returns a Date instance:
The Date.now() method returns the number of milliseconds elapsed since
January 1, 1970 00:00:00 UTC
Being a number, it doesn't have .getDate() method. But, you can make use of Date without arguments to get a Date instance which has a .getDate() method:
If no arguments are provided, the constructor creates a JavaScript
Date object for the current date and time according to system
settings.
So this should work:
new Date().getDate(); // 3

Create a Date Object with the year only

I'm used to create Date objects by using the fourth syntax from MDN as new Date(year, month, day, hours, minutes, seconds, milliseconds); But lately I tried to set a Date object with only a year (as new Date(2017)) but as you could expect it was treated as a value and considered the year as a number of milliseconds.
Is there any way of still easily use the year as is without changing the syntax and expect a correctly set Date ?

Two solutions come to my mind:
(1) Set the year argument to 2017 and set the month argument to 0 when constructing the date:
let d = new Date(2017, 0);
console.log(d.toString());
The arguments will be treated as local time; month and day of month will be January 1; all time components will be set to 0.
(2) Specify "2017T00:00" as the first and only argument when constructing the date:
let d = new Date("2017T00:00");
console.log(d.toString());
According to current specs this is a valid format and browsers are supposed to treat it as local time. The behavior is same as that of previous example.

If you are passing a single parameter (number or string), then it is taken as per doc
value
Integer value representing the number of milliseconds since January 1,
1970 00:00:00 UTC, with leap seconds ignored (Unix Epoch; but consider
that most Unix time stamp functions count in seconds).
dateString
String value representing a date. The string should be in a format
recognized by the Date.parse() method (IETF-compliant RFC 2822
timestamps and also a version of ISO8601).
Also as per doc
If at least two arguments are supplied, missing arguments are either
set to 1 (if day is missing) or 0 for all others.
You can pass one more parameter as 0 or null (or the actual value you want to set)
new Date(2017,0);
Demo
var date = new Date(2017,0);
console.log( date );

You could pass null as second argument:
new Date(2017, null);
However, without knowing the details of how missing values are interpreted, what do you think happens now? Will the date be initialized with the current month, day, hour, etc? Or something different?
Better be explicit and pass all arguments, so that you know what the code is doing half a year later.

I have another suggestion. You can just create a regular date object and set it's year. So at least you know to expect what the rest of the Date object values are.
var year = "2014";
var date = new Date();
date.setFullYear(year);
// console.log(year) => Wed Dec 27 2014 16:25:28 GMT+0200
Further reading - Date.prototype.setFullYear()

Convert date and time to UTC

I am asking the user to input a date and time into my application. The user will input a time based on the timezone they are in. When I save this date and time to my database I want to convert the time to UTC so that when I query by time (which is done in UTC) I can find the entries.
This is what I've currently done :
var date = new Date();
var dateString = "0" + (date.getMonth() + 1) + "/" + date.getDate() + "/" + date.getFullYear() + " " + time;
//format date
date = moment(dateString, "MM/DD/YYYY HH:mm a");
date = new Date(date).toISOString();
Where time is the time the user enters (ex if I want to schedule something for 11:00am, time = 11:00am)
When this is saved to the database, it looks like :
ISODate("2016-05-09T11:00:00Z") which is not correct since that is a EST saved as Zulu time.
How can I convert the time (I am using moment) to be saved as the correct Zulu time?

One option would be to use Javascript's built-in UTC functions.
JavaScript Date Reference
getUTCDate() - Returns the day of the month, according to universal time (from 1-31)
getUTCDay() - Returns the day of the week, according to universal time (from 0-6)
getUTCFullYear()- Returns the year, according to universal time
getUTCHours() - Returns the hour, according to universal time (from 0-23)
getUTCMilliseconds() - Returns the milliseconds, according to universal time (from 0-999)
getUTCMinutes() - Returns the minutes, according to universal time (from 0-59)
getUTCMonth() - Returns the month, according to universal time (from 0-11)
getUTCSeconds() - Returns the seconds, according to universal time (from 0-59)
For example,
new Date('2016-05-09 10:00:00')
returns Mon May 09 2016 10:00:00 GMT-0400
new Date('2016-05-09 10:00:00').getUTCHours()
returns 14
UPDATE: Examples (including .toISOString())
If we choose July 4, 2016 # 8:00 PM Eastern Time (GMT-0400), UTC would be July 5, 2016 # 00:00 (midnight):
var date = new Date('2016-07-04 20:00:00')
date.getUTCFullYear = 2016
date.getUTCMonth = 6 (0 base)
date.getUTCDate = 5
date.getUTCHours = 0
date.getUTCMinutes = 0
var date = new Date('2016-07-04 20:00:00')
date.toISOString() = "2016-07-05T00:00:00.000Z" (UTC)

To fix this issue I used the moment-timezone library.
I first set the timezone on the server since it will only be accessed by EST :
moment.tz.setDefault("America/New_York");
Then all I needed to do was set the timezone on the moment object to UTC by :
date = moment(dateString, "MM/DD/YYYY HH:mm a").tz("UTC");
This successfully converts the time from EST to UTC

javascript getDay() returns wrong number for a date in america but it returns correct value in india

I was trying to get the week day name using javascript getDay(). I know getDay() method returns the day of the week, like: 0 is sunday, 1 is monday etc.
var d=new Date("2014-05-26"); //this should return 1 so weekname monday.
alert(d.getDay()); // in india it returns correct value 1 fine.
But when i checked this code in USA it returns wrong number 0(sunday).
Can anybody tell me why is this happening?? I dont know where i'm doing wrong.
Thanks in advance.

Date constructor creates an instance using UTC, which differs from local time in both India and the US. You can check that by comparing
d.toLocaleString()
and
d.toUTCString()
So, what you probably need is
d.getUTCDay()
which returns the same result for all time zones.

You have two issues:
Passing a string to the Date constructor calls Date.parse, which is largely implementation dependent and differs between browsers even for the standardised parts.
ISO 8601 dates without a timezone are specified in ES5 to be treated as UTC, however some browsers treat them as local (and ES6 will change that so they should be treated as local).
So if you want consistent dates, write your own parser to turn the string into a date. Presumably you want strings without a time zone to be local, so:
function parseISODate(s) {
var b = s.split(/\D/);
var d = new Date();
d.setHours(0,0,0,0);
d.setFullYear(b[0], --b[1], b[2]);
return d.getFullYear() == b[0] && d.getDate() == b[2]? d : NaN;
}
The above function expects a date in ISO 8601 format without a timezone and converts it to a local date. If the date values are out of range (e.g. 2014-02-29) it returns NaN (per ES5). It also honours two digit years so that 0005-10-26 reuturns 26 October, 0005.
And:
parseISODate('2014-05-26').getDay() // 1
everywhere.
A simplistic version without the above (i.e. doesn't validate date values and turns years like 0005 into 1905) that can be used if you know the date string is always valid and you don't care about years 1 to 99 is:
function parseISODate(s) {
var b = s.split(/\D/);
return new Date(b[0], --b[1], b[2]);
}

try this,
var d=new Date("2014-05-26"); //this should return 1 so weekname monday.
var newDate = Date.UTC( d.getFullYear(), d.getMonth(), d.getDate());
alert(newDate.getDay()); // in india it returns correct value 1 fine.
you can produce same issue via changing your system date time zone change. set it to UTC -5/4/3 or UCT +5/4/3 and test this code

Getting current date in milliseconds (UTC) (NO use of strings)

Well, you might think that this question has already been asked, but I think it has not. The solutions I've read about all had this "jigsaw puzzle" technique (like getUTCMonth() + getUTCMinutes + ...).
But as I only want to compare the elapsed seconds between two UTC (!) dates, this does not apply.
As everybody knows, you can get the current (non-UTC) date by:
var d = new Date();
var t_millis = d.getTime();
But this is NOT what I want. I'd like to have the current system date in UTC and in milliseconds, so not mess about with strings at all. AFAIK the variable t_millis will contain the millisecond value of the current timestamp in GMT, not UTC.
(Since d is in GMT as well. Unless getTime() does a sort of implicit time zone conversion, i. e. adding the offset BEFORE giving out the milliseconds, but I've never read about that anywhere)
So is there really no other way than adding the offset to the time value?
I'm desperately missing a function like getUTCTimeMillis() known from other languages.

This is an old question but for the sake of the new visitors here is THE CORRECT ANSWER:
Date.now();
It returns the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC

The millisecond value of the time-of-day is going to be the same regardless of your time zone. That is, there are no time zones on planet Earth that differ from one another by a number of milliseconds greater than zero. (They may differ by an integer number of hours or even minutes, but not seconds or milliseconds.)
That said, the value you get back from getTime() is a UTC-relative timestamp. If two web browsers at widely different spots on the globe create a Date object at the same time, they'll both get the same value from .getTime() (assuming the clocks are synchronized, which is of course highly unlikely).
Here: 1338585185539 That's a timestamp I just got from my browser. I'm in Austin, TX, and now it's 4:13 in the afternoon (so that timestamp will be from slightly before that). Plug it into a Date instance on your machine and see what it says.
(edit — for posterity's sake, that timestamp is from 1 June 2012.)

how about:
var now = new Date();
var utc_now = new Date(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate(), now.getUTCHours(), now.getUTCMinutes(), now.getUTCSeconds(), now.getUTCMilliseconds());
console.log('UTC: ' + utc_now) // correct UTC time but wrong timezone!
console.log('UTC (in ms): ' + utc_now.getTime())

I have used this function to solve the problem.
function getUTCNow()
{
var now = new Date();
var time = now.getTime();
var offset = now.getTimezoneOffset();
offset = offset * 60000;
return time - offset;
}
The getTime function returns the number of milliseconds elapsed since
1 January 1970 00:00:00 in the client timezone.
getTimezoneOffset return offset in minutes between Client timezone and UTC.
offset = offset * 60000; this operation transform minutes in miliseconds.
subtracting the offset get the number of milliseconds elapsed since 1
January 1970 00:00:00 UTC.

To get the timestamp from a date in UTC, you need to take in consideration the timezone and daylight savings for that date. For example, a date in January or in July could mean 1 hour difference.
The option below does just that.
Date.prototype.getUTCTime = function () {
return this.getTime() - (this.getTimezoneOffset() * 60000);
};
It can be used as in:
var date = new Date();
var timestamp = date.getUTCTime();