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Dealing with js localized date in C#

My app (back-end in C# & front-end in Angular Materials) has a search screen allowing user to specify the date period using datepickers. The problem is that some of the users are not in UK while all the data they view has been created with GMT date. So if someone in Germany selects date 01/01/2017 in datepicker, my back-end reads it as 31/12/2016 23:00:00 resulting in incorrect search results.
Can someone advise me how to deal with this? I'd like to still use the Angular Material datepicker but be sure that I'm passing the date selected by the user. I know I can transform the date before posting it like this:
moment(myDate).format('MM/DD/YYYY'))
but I have a lot of cases like this and would prefer some generic solution.

For transmission and storage, I advise using UTC for everything. Only at the point of display should the time be converted to whatever locale the user has selected. Despite this being an old problem, running into time conversion issues is still quite common. Most places I've worked at will store everything as UTC timestamps or Unix epoch time with respect to UTC, that way there is no question what the meaning is anywhere in the system. If/when it needs to be rendered to something local, we do it on the client side.
For example, to get the local time converted to UTC as a string:
var noTimeZone = new Date().toUTCString();
-or-
var noTimeZone = new Date().toISOString();
Or, if you want a numeric value so you don't have to deal with funky format parsing between client/server, you can get the Unix epoch:
var unixEpochMS = new Date().getTime();
Mind you, Date.getTime() will return milliseconds rather than seconds. Also note that the Unix epoch is defined in terms of UTC. That is, any numeric value that is a timestamp is expected to be UTC. If you want a different timezone, you need to parse the value and then set the timezone to what you want.

Solution 1:
I think the solution is to get your user's timezone. You can use Javascript to get timezone from user's computer and send it to server with the request.
var d = new Date();
var tz = d.getTimezoneOffset()/-60;
tz will be 2 if user's timezone is GM+2
Soution 2:
You send and receive Unix timestamp. But then you need to convert the timestamp to readable date/time based on user's timezone.

How to tell JavaScript Date where to take TimezoneOffset from?

I am using new Date(<date-string>) and then .getTime() to pass date strings to milliseconds from 1970.
The problem is that the date strings does not contain the timezone on them. They are British, so the timezone will be GMT or GMT+1 depending on the date...
When I use this technique in the front-end (Chrome), or in the back-end (Node.js). The time zone taken is the British one (GMT or GMT+1 depending on the date). I assume that is taken from the OS.
However, when using a Node.js server which I have been told is configured to be in UTC... the timezone is always going to be GMT, leading to errors during the British Summer Time.
Is there any way to tell Date to take the timezone from the OS without changing the server configuration?
Example:
var aDate = new Date('2016-06-23 10:15:0');
var timestamp = aDate.getTime();
Just in case my explanation is not clear:
// Executed on 28-06-2016
// In the browser (in London)
new Date().getTimezoneOffset(); // -60
new Date('28-06-2016 11:11:11').getTimezoneOffset(); // -60
new Date('28-01-2016 11:11:11').getTimezoneOffset(); // 0
// In the Node.js server I am forced to use, which is configured to use UTC
new Date().getTimezoneOffset(); // 0
new Date('28-06-2016 11:11:11').getTimezoneOffset(); // 0
new Date('28-01-2016 11:11:11').getTimezoneOffset(); // 0
// Ideally, I would like to have the output I get in the browser when I run the code in the UTC Node.js server

I recommend using Moment Timezone for this, since this would be needlessly complicated to implement without a library. To get UTC in milliseconds from a given date in a given timezone, you can do this:
const moment = require('moment-timezone');
function londonTimeToUTC(dateString) {
return moment.tz(dateString, 'DD-MM-YYYY HH:mm:ss', 'Europe/London').valueOf();
}
console.log(londonTimeToUTC('28-06-2016 11:11:11')); // 1467108671000
console.log(londonTimeToUTC('28-01-2016 11:11:11')); // 1453979471000
The second argument passed to moment.tz() is a format string, which is necessary if the date string is not in ISO format. The third argument is any valid timezone identifier.

Is there any way to tell Date to take the timezone from the OS without changing the server configuration?
The time zone from the OS is what the Date object uses. If you're asking if you can change that time zone without changing the configuration, then no - there is not a way to do that. The Date object always takes on the behavior of the local time zone. Even if you supply an offset in the input string, it just uses that to determine the internal UTC timestamp. Output via most of the properties (including toString and getTimezoneOffset) will always use the local time zone.
Even in your examples, you cannot count on the browser behavior always returning the values you showed, simply because each user visiting your web site may have a different time zone setting.
The recommended way to deal with this is by using the moment.js library, which can handle UTC and local time by itself, but may require use of the moment-timezone extension if you are wanting to work with a specific time zone, such as Europe/London.
Now, with that said, if you're certain that your entire node.js application will run in a single time zone, and you're running on Linux or OSX (not Windows), then you can indeed change which time zone that node.js considers to be "local". Simply set the TZ environment variable before you launch node, like this:
env TZ='Europe/London' node server.js
There is no equivalent for the browser, or for Windows. And you still have to contend with possible non-UK users on your web site - so this doesn't guaranteed a match between client and server time. But it does address your question.
See also:
How to initialize javascript date to a particular timezone
How to make the timezone of date to UTC
How to set default timezone in Node.js

How to convert timespan using offset

I have MVC web application. I am storing UTC time in database. (Not datetime but just a time). In C# When I retrieve this time from the database I get timespan object back. I also have offset available in minutes. For example.
double offset = 600;
How do I use this offset to convert timespan to local datetime.
Note I don't want to use DateTime.ToLocalTime () method because that will use server's timezone.
UPDATE1
I am using the Javascript new Date().getTimezoneOffset() method to get the client's offset, and i have offset value stored on the server. Then I also have drop down list that show times as 12:00 AM, 12.30 AM, 1:00 AM etc etc. The dropdownlist is bound to model property SelectedDateTime of type DateTime. Idea is to convert user selected time to UTC and then UTC to localtime based on the offset. So lets say i have offset 300 minitues that would be 300/60 = 5 hours
double offset = 5.00; // this is available on the server
When the user selects time in a drop down list, I am getting a datetime object on the server, ignoring the date part i want to store UTC time into database. This is how I'm converting to UTC time.
TimeSpan utcTime = SelectedDateTime.AddHours(offset).TimeOfday;
I store this utcTime into the database. Now I want to convert UTC timespan into the client's datetime.
I am assuming i have Subtract offset now
var newLocalTimeSpan = utcTime.Subtract(TimeSpan.FromHours(offset));
var newLocalDateTime = new DateTime(newLocalTimeSpan.Ticks, DateTimeKind.Local);
However this throws the error:
Ticks must be between DateTime.MinValue.Ticks and
DateTime.MaxValue.Ticks.\r\nParameter name: ticks
For example with offest 5 hours, If user selects 8:00 PM then it will be converted to UTC and will be stored as 01:00:00.0000000 in database. When I retrieve the UTC value from database its '1:00:00 AM'. Then I subtract 5 hours form this TimeSpan which equals to `-4' now and if I pass Ticks to DateTime..i get above error.
NOTES: If you are curious why model property is DateTime instead of TimeSpan thats because i am using Kendo TimePicker which needs DateTime type.
UPDATE 2
I really appreciate all for your help. I have gone through all the articles #Matt Johnson has posted and it looks like I should not be using offset for calculating the UTC time. Mainly because of the day light time saving. But instead I should be using timezone. So I have 3 options here to find client’s time zone:
1> Use JavaScript to detect time zone
In JavaScript I can do new Date().toString() which returns date time as Sun May 22 2016 02:12:36 GMT-0500 (Central Daylight Time) I can then parse the string to get “Central Daylight Time” and post it to the server. However on server, for .net “Central Daylight Time” is not a valid windows time zone ID.
Question
Is this correct approach? Is JavaScript returning IANA zone id? Will it always return IANA zone id?
If JavaScript is returning IANA Id then I can use Matt’s article here to get windows time zone id
2> Use http://momentjs.com/ to detect client’s time zone
Question
Is momentjs returns IANA zone id?
If momentjs return IANA zone id then I can use Matt’s article above to get windows zone id. One of the reason I don’t like this approach is because I have to use 2 third party libraries momentjs and Noda Time
3> Provide user a drop down list using TimeZoneInfo.GetSystemTimeZones() and let the user selects the timezone.
User will select a time and timezone, then on server I will convert it to UTC using selected timezone and save it DB. However I have to show that time on some other pages, So I again need timezone. That means I have to put the drop down list in such a place on UI where it will be available all the time. Like top menu.
(I can certainly save timezone into DB along with the time, however if user travel to other place he will still see time in initially selected time zone. Which I don’t want)
Are these correct approaches? Am i missing something?
Question
Assume that I implement timezone selection using one of the approach above and i have correct client's time zone with windows timezone id on server in some variable.
Now lets say user selects 6:00 PM (Central Daylight Time , UTC -5) which will convert to UTC as 23:00:00. As long we are in Central Daylight Time the conversion from UTC to local will show 6:00 PM. Once we go into Central Standard Time which is UTC -6 Will the conversion still show 6:00 PM or 5:00 PM?
I am planning to use TimeZoneInfo.ConvertFromUtc(datetimevalue, timezone) method for converting UTC to Local

In general, there are only two viable approaches:
Pass only UTC dates and times to the client, and do all conversions to local time in the browser using JavaScript.
Use this approach when you don't care what the time zone actually is, but you just want it to match the browser's local time.
The Date object can do this, but you may find it easier to use a library such as moment.js, which gives you better control of output format, among other things.
Apply a time zone (not just an offset) to the UTC date and time on the server side, to produce the correct local time value.
Use this approach when the time zone affects an entire application, and needs to be known in server-side business logic.
You can try to guess the user's time zone using jsTimeZoneDetect or moment.tz.guess() in moment-timezone. However, it's just a guess, and it is always an IANA time zone ID (such as America/Los_Angeles).
Asking the user for their time zone from a list is a good idea. Usually one would place this on a user settings or profile page. You can use the guess made earlier to pick a default value from the list.
You will indeed need to use Noda Time on the server if you are using IANA time zones on the client.
Some applications choose to list Windows time zones instead, which is a much simpler approach as you can get everything from the TimeZoneInfo class. However, recognize that there are limitations with this approach including:
Localization issues, as you cannot easily get at display name strings other than the ones matching the operating system's default language, not .NET's globalization and localization features.
Maintainability issues, as you yield control to the operating system for keeping the time zone data updated. This may seem more convenient, but you may find that your hands are tied when keeping up with short-notice time zone changes. This is especially problematic when you don't have control over how or when updates are applied to the OS, such as with Microsoft Azure App Service.
Compatibility issues, as Windows time zones aren't generally recognized outside of Windows. If you ever expose the user's time zone setting in an API, you'll likely have translation issues for callers from other platforms.
Now, getting to your specific points:
I am using javascript new Date().getTimezoneOffset() method to get the client's offset...
That gives you the client's current offset. You have no guarantees that it is the correct time zone to apply for an arbitrary date and time.
If wanted to apply a fixed offset to a UTC DateTime in C#, the best way is with a DateTimeOffset.
DateTime utc = new DateTime(2016, 12, 31, 0, 0, 0, DateTimeKind.Utc);
DateTimeOffset dto = new DateTimeOffset(utc); // DateTimeKind matters here
TimeSpan offset = TimeSpan.FromMinutes(-300); // The offset is inverse of JavaScript's
DateTimeOffset result = dto.ToOffset(offset);
But do note this is only for a fixed time zone offset. For a true time zone, you would use the TimeZoneInfo class if you're using Windows time zones, or you would use NodaTime's DateTimeZone class for IANA time zones.
In JavaScript I can do new Date().toString() which returns date time as Sun May 22 2016 02:12:36 GMT-0500 (Central Daylight Time) I can then parse the string to get "Central Daylight Time" and post it to the server.
No, this approach is not recommended, for several reasons:
There's no guarantee you will get output in any particular format from JavaScript's toString function. The results are implementation specific, and will vary across browsers and platforms.
They are generally intended for display purposes. When DST is in effect, they'll show a daylight name, and when standard time is in effect they'll show a standard name.
They are often localized for the user's language, English, French, Chinese, etc.
The only native API that can return the user's time zone is:
Intl.DateTimeFormat().resolvedOptions().timeZone
This is part of the ECMAScript Internationalization API. Unfortunately, it currently only works in a handful of browsers. Both jsTimeZoneDetect and moment.tz.guess() will use this API if it's available, then will fall back to their own guessing logic if not.
Assume that i implement timezone selection using one of the approach above and i have correct client's time zone with windows timezone id on server in some variable. Now lets say user selects 6:00 PM (Central Daylight Time , UTC -5) which will convert to UTC as 23:00:00. As long we are in Central Daylight Time the conversion from UTC to local will show 6:00 PM. Once we go into Central Standard Time which is UTC -6 Will the conversion still show 6:00 PM or 5:00 PM?
I am planning to use TimeZoneInfo.ConvertFromUtc(datetimevalue, timezone) method for converting UTC to Local
As you said earlier, "Central Daylight Time" is not a valid Windows time zone identifier. Your user wouldn't pick that. You'd display a list generated from TimeZoneInfo.GetSystemTimeZones(), showing the DisplayName to the user, and using the Id for the value. The Id would be "Central Standard Time", which indeed is the correct identifier for US Central Time, inclusive of both CST and CDT - despite having the word "Standard" in the string.

You need to convert the TimeSpan to a DateTime, using the current Year, Month and Day. If you subtract from a TimeSpan without doing so, it can result in an unobtainable date.
Also, I noticed in your update that you left the results in a DateTime, so I did the same.
This code is showing you the time if the UTC time was 1:00 AM, as your problem states.
double offset = 5.00;
TimeSpan utcTime = new TimeSpan(1,0,0); //setting manually to your representation of 1 am.
DateTime newLocalDateTime = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day, utcTime.Hours, utcTime.Minutes, utcTime.Seconds);
newLocalDateTime = newLocalDateTime.Subtract(TimeSpan.FromHours(offset));

Moment.js Timezone - CONSIDER a date/time to be in a different timezone when outputting

I know this seems like a straight forward question - but although Moment.js and moment timezone are very powerful tools for getting/setting and formatting dates..
I'm having an issue snapping my application to a single timezone.
What I want, is for a user to choose from a date/time picker - and to send that as a unix timestamp in UTC.. but the date/time picker MUST be considered to be BST.
In other words, even if you're using the site from abroad - the date/time you select should be the UTC value for if you had chosen it in the UK.
var local = moment(dateTime).unix();
var london = moment(dateTime).tz('Europe/London').unix();
var berlin = moment(dateTime).tz('Europe/Berlin').unix();
All 3 variables will equal the SAME UTC timestamp on the same machine, but a different timestamp on another machine running in a different timezone.
Think about it... if my date/time was 3pm on Saturday.. that's a DIFFERENT UTC in London than it is in Berlin, since it will occur one hour earlier in Berlin.
How do I force a date/time to be considered as a specific timezone?
Thanks :-)

Assuming dateTime is an ISO8601 string, and you want the date to be in the Europe/London timezone at all times all you need is:
moment.tz(dateTime, 'Europe/London').unix()
This tells moment to interpret that time as London time, provided that it does not have a specified offset. If it has an offset, it is going to convert from the time of the offset to London time.
So, for me in America/Chicago, you can see the effect of this:
//unix timestap in london time
moment.tz('2016-12-30', 'Europe/London').unix()
1483056000
//parse that timestamp back to my local time
moment.unix(1483056000).format()
"2016-12-29T18:00:00-06:00"
Keep in mind that London has multiple offsets due to Daylight Saving Time. I think this is what you want.
If you wanted it to always keep a fixed offset of +0, then you could just use UTC:
moment.utc(dateTime)

How to calculate eastern time using Javascript Date object?

I'm working on a personal project involving Javascript, and as part of that project, I want to grab the current date (including time) and display it accordingly. No big deal right? Well, the deal is that I want to return the time & date in Eastern Daylight TIme, no matter where in the world the IP is.
If this is not possible, what alternative methods do you suggest? Does php have this functionality? I could write a simple php script that takes a date and converts it, but I want to keep this in JS if at all possible.
I'm trying to think through the best way to do this, but I'd appreciate any help that you could offer.
Thanks!

I found this on the internet, and there are a lot more of these scripts:
function calcTime(offset) {
// create Date object for current location
d = new Date();
// convert to msec
// add local time zone offset
// get UTC time in msec
utc = d.getTime() + (d.getTimezoneOffset() * 60000);
return new Date(utc + (3600000*offset));
}
So, you get the current time, add the offset of the current location to get UTC time and then you return a new date where you add the offset of a certain time zone again.

JavaScript native Date objects only know two timezones, UTC and the user's locale timezone (and even then, the amount of information you can extract about the locale timezone is limited). You could work in UTC and subtract 4 hours to get EDT, but do you really always want EDT and not EST?
If you want to do timezone conversions between arbitrary regions in PHP you'll need to drag in a large library with its own timezone information, such as TimezoneJS.
It may be better to keep the JavaScript stuff all in UTC, and let the PHP side worry about formatting it for a particular locale/timezone, using eg the timezone stuff from Date/Time.