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How to reject non UTC/GMT dates from client in C#?

I want to force clients (web, android, ios) to send the API only time in UTC/GMT.
This means that they get user's local time (using any method), convert it to UTC/GMT, and then send it to the API.
And I want to reject any datetime parameter that is not in UTC/GMT.
In JavaScript, I can get UTC this way:
new Date().toUTCString() which gives this result:
'Mon, 15 Nov 2021 04:26:38 GMT'
And I send this string to the API:
[HttpGet]
public object Parse(string clientDateTime)
{
var date = DateTime.Parse(clientDateTime);
return new
{
ParsedDate = date,
Kind = date.Kind.ToString()
};
}
However, I see that .NET parses this date time as Local and not as Utc. This is in spite of the string containing GMT.
How can I check the incoming datetime and make sure it's UTC/GMT?

You can greatly simplify your problem by using a conventional format for datetime serialization. A common choice for this problem is using the ISO 8601 datetime format.
Here you can find an in depth explanation of this format, but as an example this is a datetime in the ISO 8601 format: 2021-11-15T06:40:48.204Z (the final Z indicates that the datetime represented by this string is UTC)
The main advantage in fixing a format for date and times is that you will know in advance the format and you will be in a much better position to parse the datetime strings on the server.
Using the ISO 8601 format is a good choice, because it is a well known format and it is the standard de facto for the datetime serialization: this basically means that anyone writing a client for your application will be able to comply with the required format. Of course, you are required to clearly document this convention so that your clients (or your fellow developers) will be aware of it.
Another tip is using the DateTimeOffset struct instead of DateTime. DateTimeOffset is basically used to represent a specific point in time and that's exactly what you want: your clients will send you ISO 8601 strings representing a point in time and you want to know that point in time in your application.
Your clients will be able to use any time zone to express the point in time they want to send to your application. Doing this using an UTC datetime or any other time zone is just an implementation detail. Once you have parsed the datetime string to a DateTimeOffset instance, if you really want to, you can check whether it is an UTC time by checking the Offset property: it will be a zero TimeSpan value if the DateTimeOffset instance represents an UTC date and time.
In order to manipulate date in a Javascript client application I strongly suggest to use the Moment.js library. Check this docs to see how to get an ISO 8601 string with Moment.js
You can use this helper method to parse an ISO 8601 string to a DateTimeOffset instance. This implementation allows the client to send you a broad range of ISO 8601 compliant string formats, if you want you can be stricter by reducing the number of allowed formats (see the Iso8601Formats static field in the code).
To summarize:
ask your clients to only send you datetime strings in a format compliant with the ISO8601 specification. Clearly document this choice
for a Javascript client use a library like Moment.js to manipulate date and times. This will be much simpler than using plain old javascript Date objects.
if you are manipulating date time strings representing a specific point in time, use the DateTimeOffset struct instead of the DateTime struct. DateTimeOffset represents a specific point in time expressed in a certain time zone. The Offset property represents the difference between the point in time represented by the DateTimeOffset instance and UTC: its value will be a zero TimeSpan if the DateTimeOffset instance represents an UTC datetime. Notice that the point in time will always be the same regardless the time zone it is referring to, so using UTC doesn't make any real difference (it's just an implementation detail at this point).
use code like this one to parse a DateTimeOffset instance from a string. This code tries as many ISO 8601 compliant formats as possible (this is done in order to accept as many valid formats as possible). If you want, you can decide to be stricter: to do that, just reduce the number of formats in the Iso8601Formats array.
A final note on your code. The behaviour you are observing from DateTime.Parse is exactly the expected one. Check the documentation for DateTime.Parse:
Converts the string representation of a date and time to its DateTime
equivalent by using the conventions of the current thread culture.
DateTime.Parse is basically designed to use the locale settings of the machine running the code.
If you want to learn more on the difference between DateTime and DateTimeOffset you can check this stackoverflow question.

How do I accept multiple formats of date with moment.js

I want to accept dates from the users but I cannot enforce the users to enter the date in a specific format. I have tried going through the momentjs documentation but couldn't really find how its done. I can specify an array of formats but then I'll be tied down to those specific formats only.
I know that MS Excel and many other apps does this. (Take date input in some format and convert it to the application suitable format)
If not with moment.js how else can I achieve this? What could be the algorithm or logic that can achieve this thing.
Thanks!

Moment.js can parse an ambiguous string as a date if you don't specify expected formats. It will first attempt to parse it as an ISO 8601 date format, then as an RFC 2822 format and ultimately fall back to JS Date parsing. Keep in mind the supported formats in a JS Date are inconsistent between browsers.
For this reason it's highly advised to always parse using a set of expected formats and enforce the user to input in one of these. Otherwise you may end up with incorrectly parsed dates.

Javascript convert string to date, but ignore the timezone that is assumed it's in

A really long title, I know, but I had to highlight the fact, that I'm confronting a situation that's a little different than all the usual javascript date conversions.
I am getting the following datetime in a string from the server:
2017-05-18T08:00:00
When I put this string into the following statement:
var newDate = new Date("2017-05-18T08:00:00");
It assumes it's in the UTC timezone, so it automatically adjusts, and converts it into local time, which in Sidney would become 2017/05/18 18:00:00.
Any way that I can stop the date constructor to assume that the string is UTC time (make it assume that it's local time)?

use getTimezoneOffset() function to adjust timezone. By default Date converts it to local timezone :(

If you're applying your code in serious applications, consider a tool like Moment.js

MomentJs output Date of toDate() is incorrect

I've started using momentJs in an Angular/Typescript project. (Included incase it's relevant in any way although I very much doubt it)
In the run method of my module I call
moment.locale(window.navigator.language);
which correctly sets the locale to en-GB in my instance. Further down the line I use moment to parse a GB time.
when doing the following:
var mom = moment("24/11/2015 00:00:00");
for example. This populates a new moment object using the defaults set on the moment global (If i understand how it should work correctly). moms date is set to 2016-12-11T00:00:00.000Z. This clearly means it's parsed the given string in en-US instead of en-GB which was set via Locale in a default setting prior to this call. Is there anything I've missed in configuration/setup of moment which would make this not work?
I've also inspected the _locale property of my variable. mom._locale is set to en-gb and I can see the L,LL,LLL etc etc formats are all en-GB formatted values (as they should be).
running mom.toDate(); unsurprizingly returns the 2016 date stored internally by the moment object.
Some misc information I forgot to include:
I am using the latest release of momentjs from NuGet (Version 2.10.6 at time of writing) and I've included moment-with-locales.js in my HTML

Using any recent version of MomentJS, you should see why in the console:
Deprecation warning: moment construction falls back to js Date. This is discouraged and will be removed in upcoming major release. Please refer to https://github.com/moment/moment/issues/1407 for more info.
Unless you specify a format string, MomentJS relies on the Date object's parsing, and unfortunately, regardless of locale the Date object will, with a string using /, assume U.S. format. One of the many, many things that aren't quite right with Date.
You'll need to use a format string, or supply the string in the simplified ISO-8601 format used by Date. From Parse > String:
When creating a moment from a string, we first check if the string matches known ISO 8601 formats, then fall back to new Date(string) if a known format is not found.
var day = moment("1995-12-25");
Warning: Browser support for parsing strings is inconsistent. Because there is no specification on which formats should be supported, what works in some browsers will not work in other browsers.
For consistent results parsing anything other than ISO 8601 strings, you should use String + Format.

So I got around this by fetching the locale data from moment and just passing it into the format parameter. Considering the example input of "24/11/2015 00:00:00" I would structure my format as below:
var format = moment.localeData().longDateFormat('L') + " " + moment.localeData().longDateFormat("LTS");
this generates the format mask of "DD/MM/YYYY HH:mm:ss".
You can mix and match whatever formats you want and this will be locale specific to whatever you set moment.locale("") to be (presuming you have the locale information setup in moment already)
This is a crazy workaround and I'm surprised that moment doesn't presume locale information as default when parsing. TJCrowder has raised an issue on Github with the moment guys which I suggest anyone who cares should comment on. https://github.com/moment/moment/issues/2770

You're probably better off passing the format to moment directly and validating the string before hand. This will ultimately reduce the amount of debugging you'll need to do and get you up and running straight away.
var mom = moment("24/11/2015 00:00:00", "DD/MM/YYYY HH:mm:ss");
You could try the new(ish) Intl API but browser support is limited (IE11+), so I would recommend having a user select the month in a dropdown or something to force them to input a certain way.

formatting ISO date

I have making an API call and getting response.
The response has datetime value like this
20131107T102103,00+01
I guess this is ISO date format. I am not able to format it to human readable format using javascript.

This is indeed ISO 8601, but it's not exactly the most common form of it.
This is the compact form. On the web, we usually see the extended form, which is also described in RFC 3339.
It's using a comma instead of a decimal in the seconds field. While either are allowed by the ISO 8601 specification, most implementations use a decimal point.
While ISO 8601 allows for a shortened form of the offset (+01), typically this is expressed in the compact form as +0100, or in the extended form as +01:00.
Nonetheless, the value is ISO 8601 compliant, so it should be parseable by most newer browsers. But it's not.
Internet Explorer 11
Google Chrome 35
Firefox 30
So, what to do then? You can write a bunch of code to pull out all the individual parts of the string and build a Date object yourself, but that's too much work. Just use moment.js. This is one of the leading libraries for parsing and formatting date and time in JavaScript, and it works in all browsers.
// parse it to a moment object first
var s = "20131107T102103,00+01";
var m = moment(s + "00","YYYYMMDD[T]HHmmss[,]SSZZ");
// then you can output however you like
m.format("...whatever...")
// or perhaps you need a JS Date object
m.toDate()
Notice that I still had to add two extra zeros to the offset. Even moment.js doesn't recognize offsets with only two digits.

date_default_timezone_set('America/New_York');
$d1=new DateTime('20131107T102103,00+01');
$date = DATE_FORMAT($d1,'Y-m-d H:i:s');