UTC date and time to local - javascript

I write a function to reverse UTC time to local time
function utcToLocal(utc){
var t = new Date(Number(utc));
d = [t.getFullYear(), t.getMonth(), t.getDate()].join('/');
d += ' ' + t.toLocaleTimeString();
return d;
}
but i can't confirm this code is right?

You should be able to convert the UTC timestamp into a local date and just subtract the local offset (which is in minutes), so:
function utcToLocal(utc){
// Create a local date from the UTC string
var t = new Date(Number(utc));
// Get the offset in ms
var offset = t.getTimezoneOffset()*60000;
// Subtract from the UTC time to get local
t.setTime(t.getTime() - offset);
// do whatever
var d = [t.getFullYear(), t.getMonth(), t.getDate()].join('/');
d += ' ' + t.toLocaleTimeString();
return d;
}
Where I am, the offset is -600, so I need to subtract -36,000,000 ms from UTC time (which actually adds 36,000,000 ms).
Edit
I may have misunderstood the question.
The internal value of a javascript date instance is a UTC time clip in milliseconds. So if utc is such a time (e.g. 2012-08-19T00:00:00Z is 1345334400000), then the OP will create a date instance based on that value and toLocaleTimeString will show an implementation dependent string of the local time for the supplied UTC time.
So if the local timezone offset is say -6hrs, then alert(new Date(1345334400000))) show something like Sat Aug 18 2012 18:00:00 GMT-600.
I was assuming that the OP wanted to set the local time to the same time as the UTC time, e.g. that 2012-08-19T00:00:00Z would become 2012-08-19T00:00:00 local.

Related

Time zone issue involving date fns format()

const dt = new Date('2017-12-12');
console.log(format(dt, 'YYYY-MM-DD'));
The above code logs 2017-12-11 in the US, but 2017-12-12 in India.
I followed this github thread here and tried out things but am not getting the desired results.
My expectation is to print the same date irrespective of time zone
Why I need this :
Consider a scenario involving birthdates. If i am giving some input date, it has to be displayed as same date in all regions irrespective of their timezones.
You will need to subtract the time zone offset of your local time zone from the Date instance, before you pass it to format from date-fns. For example:
const dt = new Date('2017-12-12');
const dtDateOnly = new Date(dt.valueOf() + dt.getTimezoneOffset() * 60 * 1000);
console.log(format(dtDateOnly, 'YYYY-MM-DD')); // Always "2017-12-12"
Problem
You want to handle only the date part of the Date instance, because the time part does not make sense for birthdates. However, the Date object does not offer any "date-only" mode. You can access both its date and time parts in the local time zone or UTC. The problem is, that format from date-fns prints the output always in the local time zone.
When you executed the constructor only with the date part:
const dt = new Date('2017-12-12');
The JavaScript engine actually assumed a string in the incomplete ISO 8601 format and perfomed this:
const dt = new Date('2017-12-12T00:00:00.000Z');
It may still look "harmless" to you, but the date instance exposes the value not only in UTC, but also in the local time zone. If you construct the Date instance on the East Coast of the US, you will see the following output:
> const dt = new Date('2017-12-12');
> dt.toISOString()
'2017-12-12T00:00:00.000Z'
> dt.toString()
'Tue Dec 11 2017 19:00:00 GMT-0500 (EST)'
> d.toLocaleString()
'12/11/2017 7:00:00 PM'
Solution
If you know, that format from date-fns reads date and time parts from the date instance in the local time zone, you will need to make your date "looking like" the midnight in your local time zone and not in UTC, which you passed to the Date constructor. Then you will see the year, month and date numbers preserved. It means, that you need to subtract the time zone offset of your local time zone for the specified day. Date.prototype.getTimezoneOffset returns the offset, but with an inverted sign and in minutes.
const dt = new Date('2017-12-12');
// Tue Dec 11 2017 19:00:00 GMT-0500 (EST)
const dtDateOnly = new Date(dt.valueOf() + dt.getTimezoneOffset() * 60 * 1000);
// Tue Dec 12 2017 00:00:00 GMT-0500 (EST)
console.log(format(dtDateOnly, 'YYYY-MM-DD'));
// Prints always "2017-12-12", regardless the time zone it executed in
However, such Date instance can be used only to format the date-only value. You cannot use it for computing date differences, for example, which would need the original and correct UTC value.
Alternative
If you need always the same date-only format and not the format specific to the current locale, you do not need date-fns. You can format the string by the concatenation of padded numbers:
const dt = new Date('2017-12-12');
const year = dt.getUTCFullYear()
const month = dt.getUTCMonth() + 1 // Date provides month index; not month number
const day = dt.getUTCDate()
// Print always "2017-12-12", regardless the time zone it executed in
console.log(year + '-' + padToTwo(month) + '-', padToTwo(day));
// Or use a template literal
console.log(`${year}-${padToTwo(month)}-${padToTwo(day)}`);
function padToTwo (number) {
return number > 9 ? number : '0' + number
}
Only adding the #ferdinand-prantl answer. If you are using the date-fns, you can parse the string date ('2017-12-12') using the parseISO(here) fn from date-fns, which will complete the missing ISO 8601 format with your local time zone. When you use the format fn, you are going to keep the date.
const strDate = '2017-12-12';
const isoDate = parseISO(strDate);
const formattedDate = format(isoDate, 'YYYY-MM-DD');
console.log({strDate, isoDate, formattedDate})
//{
// strDate: '2017-12-12',
// isoDate: 2017-12-12T02:00:00.000Z,
// formattedDate: '2017-12-12'
//}

How to get hours and minutes in desired timezone without creating new moment object?

I have to display a string on the web page in this format: 16:00 HH:mm
I'm using a moment object to represent a date/time and timezone.
var day = moment().tz('GMT');
day.hours(16).minutes(0).seconds(0).milliseconds(0);
So this is 16:00 in GMT time.
On my web page I want to change the time zone and then collect the hours and minutes.
If I make a new moment object
var day2 = moment().tz('PST); //this is 8 AM since gmt was 16
console.log(day2.get('hours'));
it is 16 not 8!
and try to get the hours and minutes they are in GMT not in PST.
How can I get it in PST? Do I have to keep wrapping it?
// initialize a new moment object to midnight UTC of the current UTC day
var m1 = moment.utc().startOf('day');
// set the time you desire, in UTC
m1.hours(16).minutes(0);
// clone the existing moment object to create a new one
var m2 = moment(m1); // OR var m2 = m1.clone(); (both do the same thing)
// set the time zone of the new object
m2.tz('America/Los_Angeles');
// format the output for display
console.log(m2.format('HH:mm'));
Working jsFiddle here.
If you can't get it to work, then you haven't correctly loaded moment, moment-timezone, and the required time zone data. For the data, you either need to call moment.tz.add with the zone data for the zones you care about, or you need to use one of the moment-timezone-with-data files available on the site.
In the fiddle, you can see the moment-files I'm loading by expanding the External Resources section.
PST can mean different things in different regions. In the moment-timezone docs, I see nothing referring to "PST" or similar abbreviations.
Perhaps try:
var day2 = moment().tz('PST');
// 16 with Error: Moment Timezone has no data for PST. See http://momentjs.com/timezone/docs/#/data-loading/.
var day2 = moment().tz('America/Los_Angeles');
// 15
I don't know about using moment.js, but it's fairly simple using POJS and the same algorithm should work. Just subtract 8 hours from the UTC time of a date object and return a formatted string based on the adjusted UTC time.
Assuming PST is "Pacific Standard Time", also known as "Pacific Time" (PT), and is UTC -8:00:
/* #param {Date} date - input date object
** #returns {string} - time as hh:mm:ss
**
** Subtract 8 hours from date UTC time and return a formatted times string
*/
function getPSTTime(date) {
var d = new Date(+date);
d.setUTCHours(d.getUTCHours() - 8);
return ('0' + d.getUTCHours()).slice(-2) + ':' +
('0' + d.getUTCMinutes()).slice(-2) + ':' +
('0' + d.getUTCSeconds()).slice(-2);
}
document.write('Current PST time: ' + getPSTTime(new Date));
There is moment-timezone which adds functionality to moment.js for IANA time zones. For PST you can use America/Los_Angeles, however it might also automatically adjust for daylight saving so you'll get PDT when that applies. If you want ignore daylight saving, use the above or find a location with the offset you need and use that.

Server's time zone to be considered when opening application's UI from client machine

Our application is built using Java where we get Server time using datetime object. But when used from javascript I can't use the value as it is because value sent for UI is in string format (I don't know whether JSON attribute value retains date type in ajax when used from UI) and I want to use that value for pointstart of Highcharts and it expects the value to be of type Number.
So when I get value from REST, date will be in json format and type will be of type string. If it is converted to Date object, then it is converted back to Client's machine.
I have seen many suggestions and one among those is adding offset for the local time and I picked this for solving my problem(refer Convert date in one timezone to another timezone using timezone abbreviations for details on the solution). But in this at last it is converted to local string which is again not considered from HighCharts for pointstart. If I remove toLocaleString from Date object and try, the value what is receive is as below
"Tue Oct 20 2015 23:05:37 GMT+0530 (IST)" where Date value Tue Oct 20 2015 23:05:37 (server time) is of PDT time but time zone value appended is "GMT+0530 (IST)" (client time zone). Value is correct but can't consider as client time zone is appended. What's wrong in this?
please let me know
Code snippet:
// create Date object for current location
var date = new Date();
// convert to msec
// add local time zone offset
// get UTC time in msec
var utc = date.getTime() + (date.getTimezoneOffset() * 60000);
// create new Date object for different city
// using supplied offset
var newDate = new Date(utc + (3600000 * offset));
// return time as a string. But im removing this since i dont want of type string
return "The local time in " + city + " is " + newDate.toLocaleString();

javascript Date timezone issue

I need a js Date object with specified values for date and year. I would expect
new Date("2000-01-01") to give me Date object with 2000 as value for getFullYear(), but if my computer's time settings are set to Chicago timezone, I'm getting Fri Dec 31 1999 18:00:00 GMT-0600 (CST), and for Buenos Aires: Fri Dec 31 1999 22:00:00 GMT-0200 (ARST).
Is there a way to create Date object, with .getFullYear() returning the date we set in constructor, no matter what timezone is set on user's machine?
Update:
I need this Date object to be used in another library (which calls its .getFullYear() method, so using UTC getters doesn't really help.
When parsing a string to a Date in JavaScript, a value that is in YYYY-MM-DD format is interpreted as a UTC value, rather than a local-time value.
The key is that the parts are separated by hyphens, and that there is no time zone information in the string. The ECMAScript 5.1 Spec says in §15.9.1.15:
... The value of an absent time zone offset is “Z”.
That means, if you don't specify an offset, it will assume you meant UTC.
Note that since this is the opposite of what ISO-8601 says, this is behavior has been changed in ECMAScript 2015 (6.0), which says in §20.3.1.16:
... If the time zone offset is absent, the date-time is interpreted as a local time.
Therefore, when this provision of ES6 is implemented properly, string values of this format that used to be interpreted as UTC will be interpreted as local time instead. I've blogged about this here.
The workaround is simple. Replace the hyphens with slashes:
var s = "2000-01-01";
var dt = new Date(s.replace(/-/g, '/'));
Another workaround that is acceptable is to assign a time of noon instead of midnight to the date. This will be parsed as local time, and is far enough away to avoid any DST conflicts.
var s = "2000-01-01";
var dt = new Date(s + "T12:00:00");
Alternatively, consider a library like moment.js which is much more sensible.
var s = "2000-01-01";
var dt = moment(s, 'YYYY-MM-DD').toDate();
You can write new method to 'Date.prototype', and use it to get date which will be including the local timezone offset.
//return the date with depend of Local Time Zone
Date.prototype.getUTCLocalDate = function () {
var target = new Date(this.valueOf());
var offset = target.getTimezoneOffset();
var Y = target.getUTCFullYear();
var M = target.getUTCMonth();
var D = target.getUTCDate();
var h = target.getUTCHours();
var m = target.getUTCMinutes();
var s = target.getUTCSeconds();
return new Date(Date.UTC(Y, M, D, h, m + offset, s));
};
Here is a little trick that may help someone:
let date = new Date();
console.log(date); // -> Fri May 28 2021 01:04:26 GMT+0200 (Central European Summer Time)
const tzOffsetMin = Math.abs(date.getTimezoneOffset()) // the minutes of the offset timezone
const tzOffsetHour = tzOffsetMin / 60; // timezone offset in hour
console.log(tzOffsetHour); // -> 2
date.setHours(date.getHours() + tzOffsetHour); // sum to date hour the timezoneoffset
const isovalue = date.toISOString();
console.log(isovalue); // -> 2021-05-28T01:04:26.156Z
In this way you "bypass" the timezone offset wherever you are

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();

Categories

Resources