Java Script Millisecond Issues - javascript

As w3schools say, Date.parse() returns "the number of milliseconds between the date string and midnight of January 1, 1970."
Which means that
if I write Date.parse("January 1, 1970 00:00:00"), it should give me answer 0.
if I write Date.parse("January 1, 1970 00:00:05"), it should give me answer 5000.. But Im getting the -14395000... Why is that?

You don't specify a time zone so January 1, 1970 00:00:00 is with the time offset of your timezone (or more precisely the one the browser has chosen for you). The milliseconds that are returned are relative to to the UTC.
MDN Date.parse:
The Date.parse() method parses a string representation of a date, and returns the number of milliseconds since January 1, 1970, 00:00:00 UTC.
[...] If you do not specify a time zone, the local time zone is assumed. GMT and UTC are considered equivalent. The local time zone is used to interpret arguments in RFC2822 Section 3.3 format that do not contain time zone information. [...]

Related

Date.now official format name

I read the documentation for the Date.now() method.
It's understood what it returns, and my question focus on, literally, the format name.
I have a REST call, where on of its properties is expirtyDate, and I wish to format to be as returned by the Date.now() method (i.e- the numbers of milliseconds passed from Jan 1970).
But how can I describe this format the my teammates? I can't tell them "use the Date.now() of JS". I prefer to ask them for "use time format"
So how should I call this format?
From the documentation you link to:
the number of milliseconds elapsed since 1 January 1970 00:00:00 UTC
or
the milliseconds elapsed since the UNIX epoch
AKA UNIX time, POSIX time, and epoch time).

Why isn't "2016-02-16" equal to "2016-02-16 00:00"?

I'm trying to pass both date strings to new Date(t).
I expect both strings represent the same time, after all, if I omit the time, shouldn't it be midnight of that day?
But while,
new Date("2016-02-16 00:00")
returns 2016-02-16, midnight, local time as expected,
new Date("2016-02-16")
returns 2016-02-16, midnight UTC, which is wrong, or at least not what I expected given what the other string parses as.
I would understand it if they would both have the same behavior, whether it is to return the time as local time, or as UTC, but it seems very inconsistent why they return different things like this.
As a workaround, whenever I encounter a date that has no corresponding timestamp I can append " 00:00" to get consistent behavior, but it seems like this is rather fragile.
I am getting this value from an INPUT element, of type 'datetime-local' so it seems especially inconsistent that I have to work around a value returned by a page element.
Am I doing something wrong, or should I be doing something differently?
It's what the ES5.1 specification says to do:
The value of an absent time zone offset is “Z”.
It also says:
The function first attempts to parse the format of the String according to the rules called out in Date Time String Format (15.9.1.15). If the String does not conform to that format the function may fall back to any implementation-specific heuristics or implementation-specific date formats.
Since the format requires a T separator between date and time, the valid times go to UTC:
> new Date("2016-02-16T00:00:00")
Tue Feb 16 2016 01:00:00 GMT+0100 (CET)
> new Date("2016-02-16")
Tue Feb 16 2016 01:00:00 GMT+0100 (CET)
...while in node.js, an invalid time (without the T separator) seems to go to the implementation specific localtime:
> new Date("2016-02-16 00:00:00")
Tue Feb 16 2016 00:00:00 GMT+0100 (CET)
Note that ES6 changed this, in the same part of the documentation it changes to:
If the time zone offset is absent, the date-time is interpreted as a local time.
The joy of breaking changes.
Edit
According to TC39, the specification is meant to be interpreted as date and time strings without a time zone (e.g. "2016-02-16T00:00:00") are treated as local (per ISO 8601), but date only strings (e.g. "2016-02-16") as UTC (which is inconsistent with ISO 8601).
According to the specifications:
The function first attempts to parse the format of the String
according to the rules called out in Date Time String Format
(15.9.1.15). If the String does not conform to that format the
function may fall back to any implementation-specific heuristics or
implementation-specific date formats.
And Date Time String Formats accept 2016-02-16 as a valid date
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
[...] If the HH, mm, or ss fields are absent “00” is used as the value
and the value of an absent sss field is “000”. The value of an absent
time zone offset is “Z”.
Thus 2016-02-16 translates to 2016-02-16T00:00:00.000Z.
The other date 2016-02-16 00:00 does not conform to the format and therefore its parsing is implementation specific. Apparently, such dates are treated as having local time zone and your example date will return different values depending on time zone:
/* tz = +05:00 */ new Date("2016-02-16 00:00").toISOString() // 2016-02-15T19:00:00.000Z
/* tz = -08:00 */ new Date("2016-02-16 00:00").toISOString() // 2016-02-16T08:00:00.000Z
Summary:
For conforming date time formats the behavior is well defined — in the absence of time zone offset the date string is treated as UTC (ES5) or local (ES6).
For non-conforming date time formats the behavior is implementation specific — in the absence of time zone offset the usual behavior is to treat the date as local.
As a matter of fact, the implementation could choose to return NaN instead of trying to parse non-conforming dates. Just test your code in Internet Explorer 11 ;)
You are perhaps running into a differences between ES5, ES6 implementations and your expected result. Per Date.parse at MDN, "especially across different ECMAScript implementations where strings like "2015-10-12 12:00:00" may be parsed to as NaN, UTC or local timezone" is significant.
Additional testing in Firefox 44 and IE 11 revealed they both return a date object for new Date("2016-02-16 00:00"), which object returns NaN when atttempting to get a date component value, and whose toString value is
"Invalid Date" (not "NaN"). Hence appending " 00:00 to get consistent behavior" can easily break in different browsers.
As noted in other answers new Date("2016-02-16") uses a timezone offset of zero by default, producing midnight UTC instead of local.
Per DateParser::Parse() of V8 source codes for Chrome.
ES5 ISO 8601 dates:
[('-'|'+')yy]yyyy[-MM[-DD]][THH:mm[:ss[.sss]][Z|(+|-)hh:mm]]
An unsigned number followed by ':' is a time value, and is added to the TimeComposer.
timezone defaults to Z if missing
> new Date("2016-02-16 00:00")
Tue Feb 16 2016 00:00:00 GMT+0800 (China Standard Time)
A string that matches both formats (e.g. 1970-01-01) will be parsed as an ES5 date-time string - which means it will default to UTC time-zone. That's unavoidable if following the ES5 specification.
> new Date("2016-02-16")
Tue Feb 16 2016 08:00:00 GMT+0800 (China Standard Time)
returns 2016-02-16, midnight UTC, which is wrong, or at least not what I expected given what the other string parses as.
It adds the timezone offset to the 00:00
new Date("2016-02-16") outputs Tue Feb 16 2016 05:30:00 GMT+0530 (India Standard Time)
My timezone being IST with an offset value (in minutes) +330, so it added 330 minutes to 00:00.
As per ecma-262, section 20.3.3.2 Date.parse ( string )
If ToString results in an abrupt completion the Completion Record is
immediately returned. Otherwise, parse interprets the resulting String
as a date and time; it returns a Number, the UTC time value
corresponding to the date and time. The String may be interpreted as a
local time, a UTC time, or a time in some other time zone, depending
on the contents of the String.
When you explicitly set the time-units new Date("2016-02-16 00:00") it wll use set that as hours and minutes,
Otherwise as stated here in 20.3.1.16
If the time zone offset is absent, the date-time is interpreted as a
local time.

Javascript getFullYear() Date method weird behavior

Can anyone explain why getFullYear does not return 2014?
console.log(new Date('2014-01-01').getFullYear()) //2013
console.log(new Date('2014-01-01').getUTCFullYear()) //2014
From MDN:
The dateString of "March 7, 2014" returns a different date than "2014-03-07" unless the local time-zone is UTC. When converting a dateString of "March 7, 2014" the local time-zone is assumed. When converting a dateString of "2014-03-07" the UTC time-zone is assumed. This results in two different Date values depending on the format of the string that is being converted.
So when you ask it to parse "2014-01-01", you're getting the time in UTC.
Then you call .getFullYear() on your object, which uses local time. If you live in the Eastern US like I do, then it basically subtracts 4 hours from the internal time and returns the year.
So here's what happens:
"2014-01-01" is converted to "1388534400000"
.getFullYear() is called and "1388534400000" is converted to local time
Local time is something like "1388534160000"
New years hasn't yet occurred at "1388534160000", so it's still 2013
All of this implies that if we do something like
console.log(new Date('January 1, 2014').getUTCFullYear()); // 2014
console.log(new Date('January 1, 2014').getFullYear()); // 2014
We'll get the same year, because we told the browser to use our timezone right on New Year's, but they're not equivalent:
console.log(new Date('January 1, 2014').getUTCHours()); // 5
console.log(new Date('January 1, 2014').getHours()); // 0
According to this:
"The difference is when you specify a string in the format YYYY-MM-DD, you get a date that is 12am in the GMT timezone and when you specify a date in the format DD-MM-YYYY, you get a date that is 12am in your current timezone."
So basically since you are specifying News Years Day 2014 when it gets converted from GMT to your local time it believes it is 12-31-13 not 01-01-14.

Retrieving the timezone offset that was already input

I create a date like this:
var date = new Date('Wed, 19 Mar 2014 18:17:00 +0200');
This resolves to:
Wed Mar 19 2014 17:17:00 GMT+0100 (Central European Standard Time)
Is there a way to retrieve the "+0200" portion from the date object once it is created? I am trying to get this without parsing the input string and without the use of external libraries.
EDIT:
When I use
date.getTimezoneOffset();
It returns "-60", which corresponds to the local timezone offset, which in my case is GMT+0100. The question I am asking is whether the "+0200" from the input is lost in the Date object upon creation, or is it stored somewhere?
You can retrieve the timezoneoffset with date.getTimezoneOffset(); and use that to calculate it
From the Mozilla MSDN
The getTimezoneOffset() method returns the time-zone offset from UTC, in minutes, for the current locale.
date.getTimezoneOffset()
The time-zone offset is the difference, in minutes, between UTC and local time. Note that this means that the offset is positive if the local timezone is behind UTC and negative if it is ahead. For example, if your time zone is UTC+10 (Australian Eastern Standard Time), -600 will be returned. Daylight saving time prevents this value from being a constant even for a given locale.
A date object is stored as the number of milliseconds since the Unix epoch. So your input string is parsed and stored as a primitive number. So no, you can't retrieve aspects of your original input after it has been converted to a date object.

timezone issue with Date.Parse()

I have the following piece of code in my script:
Date.parse('10/01/2010 01:01:01')
The result I get after running the script is this:
September 30, 2010 22:01:01
Is it maybe a problem with Date.Parse() or am I doing something wrong?
It's not a problem: it's a feature:
The parse method takes a date string (such as "Dec 25, 1995") and returns the number of milliseconds since January 1, 1970, 00:00:00 UTC. The local time zone is used to interpret arguments that do not contain time zone information. [...] If you do not specify a time zone, the local time zone is assumed.
... so there's a difference between:
Date.parse("Thu, 01 Jan 1970 00:00:00");
// ... returns 14400000 in timezone GMT-0400, and other values in other
// timezones, since there is no time zone specifier in the argument.
... and...
Date.parse("Thu, 01 Jan 1970 00:00:00 GMT-0400");
// ... returns 14400000 no matter the local time zone.

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