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How to localize input from an html datetime-local input

I used a standard html datetime-local input like <input type="datetime-local" />, I put in a time of February 16th, 6 pm from my pc in the EST GMT-5 timezone. From another pc in the same timezone I display the date stored in DB from the input, but it shows February 16th, 1 pm instead of 6 pm. I know this is a timezone problem because there is a 5 hour difference and im in GMT-5. How would I convert the users input to UTC time based on their local time, then store it the db as a UTC time?
EDIT: I did new Date(...).toUTCString() on my server instead of on my client, does this have anything to do with the timezone problem?

(1) the data we get from datetime-local input is like this
const startTime = "2021-08-28T09:00"
There is no timezone.
The MDN link here:
datetime-local
(2):
When we send the data to the server (the server timezone may different in different environment) and save in the DB(normally always in UTC timezone) we need to make sure we send the right date time. Convert it to UTC time before sending to server.
We can do:
let result = new Date(startTime);
result.toISOString();
It converts to UTC. Then it will save in the DB as UTC.
Link here: toISOString()
(3):
We get the UTC data from DB, when it display on the FrontEnd, we need to convert to our local timezone and right format as below.
this.startTime = moment(this.startTime).format("YYYY-MM-DDTHH:mm");

Info here https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/datetime-local
<input type="hidden" id="timezone" name="timezone" value="-05:00">

Preserve Local DateTime in Javascript

My customer's store is in GMT +5:30 timezone but the user's locale is in GMT +8 timezone.
Currently, I'm using javascript's .toISOString() function to convert to UTC and storing UTC in the database. I retrieve UTC from the database and send exactly that the browser, so the new Date('2019-11-15T00:00:00Z') function converts the UTC to the browser's locale.
But, if the user opens a record created by GMT +8 timezone user or vice-versa, the dates are getting messed up.
I'm thinking it would be good if I can transfer the exact date the user enters in the browser and send that exact date to the backend to easily offset using the store's timezone?
The frontend is in VueJs and the backend is in C#.

Always store UTC time in database
Just Store the UTC time in your database, In your client-side, Get client's current timezone,
Here, I am getting IANA timezone from client's system
// get client's timezone (From user's system)
var clientTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
Then, Convert the UTC depends on the timezone and show it to user,
// assume 2019-11-15T00:00:00Z is the UTC date from your database
var convertedTime = new Date('2019-11-15T00:00:00Z').toLocaleString("en-US", {timeZone: clientTimezone});
Example
var clientTimezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
var timeUtc = '2019-11-15T00:00:00Z';
var convertedTime = new Date(timeUtc).toLocaleString("en-US", {timeZone: clientTimezone});
console.log("Time to Current User : " + convertedTime);
var timeInUsa = new Date(timeUtc).toLocaleString("en-US", {timeZone: "America/New_York"});
console.log("Time in America (New York) : " + timeInUsa);
var timeInAustralia = new Date(timeUtc).toLocaleString("en-US", {timeZone: "Australia/Brisbane"});
console.log("Time in Australia (Brisbane) : " + timeInAustralia);

Note that the js Date object is simply a number that represents an absolute time (independent of any timezone).
So the best encoding/format for transferring and storing a date is that number. IMO this is much simpler than storing a UTC string.
So, on the end-user's machine you would Date.parse the date string provided by the user and this would take account of the user's time zone for you and give you the absolute time number for sending and storing on your backend.
Don't do any formatting or parsing of dates on the backend if you are using Node because there are serious gotcha's and because you shouldn't need to anyway: any client device that needs a date string will do the formatting locally which will automatically convert it to the correct format for their locale and timezone. [edit: the Q did not specify the backend when I wrote this.]
You will need to watch for some gotcha's in the Date.parse function but these are minor compared to the node problems. The most significant IMO is that it will interpret dates in YYYY-MM-DD as ISO 8601 dates (which makes some sense) but then assume that they are GMT if no timezone is specified so you should make sure there is a timezone specified if you use that format.

How to save correct time in database?

I have one object called appointment which has two properties: StartDate and EndDate.
When I make POST request I send these values using ISOString time .
this.appointment.StartDate.toISOString()
On the server-side, I received these properties with correct values. Also, it seems to be correct when I create model in order to save appointment to the database. I used .ToUniversalTime() method.
var newAppointment = new Appointment()
{
StartDate =Convert.ToDateTime(model.StartDate).ToUniversalTime(),
EndDate = Convert.ToDateTime(model.EndDate).ToUniversalTime(),
SpecialityId = speciality.Id,
LocationId = location.Id,
PatientId = patient.Id,
UserId = user.Id,
Observations = model.Observations
};
But in database I found another values. Can explain somebody why is this behaviour ?
For instance, I used 2017.09.01 11:00 for StartDate and in database i found 2017-09-01 08:00
The server and database is located in the westeurope.

A few things:
Don't call ToUniversalTime in a web application. It's designed to convert from the server's local time zone to UTC. The server's time zone should be irrelavent to your application. Web applications should never use ToUniversalTime, ToLocalTime, DateTime.Now, TimeZoneInfo.Local, DateTimeKind.Local or any other method that uses the time zone of the computer it's running on.
Ideally, on the server side, your model.StartDate and model.EndDate would already be DateTime objects, because they'd have been deserialized that way. Therefore, you probably don't need to call Convert.ToDateTime. If they are strings, then I would adjust your model class accordingly.
On the client side, assuming StartDate and EndDate are JavaScript Date objects, and they were created using local time values (that is, the time zone of the browser), when you call toISOString, you're not just getting a string in ISO 8601 format - it is also converting it from the browser's time zone to UTC.
In your example, the UTC time is 3 hours ahead of UTC for the date and time shown. From your profile, I see you are located in Romania, which is indeed UTC+3 for this date, because it is currently observing Eastern European Summer Time. When Summer Time ends (on October 29, 2017 at 04:00), it will return to UTC+2. For this reason, you cannot simply add three hours to all values.
If you want to send local time values from the client, you should send them in ISO 8601 format, without any Z or offset, for example 2017-09-01T11:00. There are several ways to achieve this:
The best way is to not have them in a Date object to begin with. For example, if your input uses the <input type="datetime-local" /> input type (as specified in HTML5), the .value property is not a Date object, but rather a string in ISO 8601 format.
If you can't avoid a Date object, then create a local ISO string, like this:
function dateToLocalISOString(date) {
var offset = date.getTimezoneOffset();
var shifted = new Date(date - offset * 60 * 1000);
return shifted.toISOString().slice(0, -1);
}
OR, using Moment.js:
moment(yourDateObject).format("YYYY-MM-DD[T]HH:mm:ss.SSS")
Lastly, you will probably read advice from others about storing these as UTC. Don't listen. The advice "always use UTC" is shortsighted. Many scenarios require local time. Scheduling appointments is a primary use case for local time. However, if you need to act on that appointment, you'll use the current UTC time, and you'll also need some information about the time zone for the appointment so you can convert from UTC to the appointment's time zone. For example, if this is something like an in-person doctor's office appointment, then it's safe to assume the time zone of the doctor's office. But if it's an appointment for an online meeting, then you'll have to capture the user's time zone separately and apply it on the back end where appropriate.

The problem is with your current timezone.
What your application does is get current timezone (+3) in this case.
Now it got your timezone but it will convert to utc time. So what will happen, your current time will be -3 hours.
If you not adjust to summer and winter time then you can simply add 3 hours to the datetime. Otherwise you have to get the offset of your timezone and add that to the current datetime value.
Take care if you use this application in different timezones. For example You life in +3 and some else life in +2 timezone.

Javascript - displaying locale time from mySQL TimeDate

I am having an issue with displaying the correct time. I have a php script that when a button is clicked it inserts the CURRENT_TIMESTAMP into the database. The server is located in Arizona, I am in PST. When I call the time in my script it shows Arizona time, but I need it to show the users time. So 2015-02-18 16:06:28 Arizona time, MY time is 2015-02-18 15:06:28.
How do i get the correct time. I am using moment.js, but no matter how i format it it shows the incorrect time. I am not sure but is DST, not being considered?
var time_in = time_in;//format 2015-02-18 16:17:33
var timeIn = moment.utc(time_in, "HH:mm a").format("HH:mm a");

Moment.js parses the date as a locale date-time. So when you do moment.utc(time_in), you're converting it to UTC according to your local time (PST), shifted forward or backwards.
So what you need to do is do a moment.fn.utcOffset. Arizona is UTC-07:00, so we would want to add +7 to the offset. You can do the same using moment.fn.zone but that's getting deprecated.
var utcTime = moment.utc('2015-02-18 16:06:28').utcOffset(+7).format('YYYY-MM-DD HH:mm:ss')
// returns "2015-02-18 23:06:28" which is the UTC time
Now you have the moment in UTC, you can convert it to the client localtime:
moment(moment.utc(utcTime).toDate()).format('YYYY-MM-DD HH:mm:ss')
// returns '2015-02-18 15:06:28' (which is PST)
moment.utc(utcTime).toDate() above just converts the utc time to your local time, then formatting it with momentjs
EDIT: If possible, you should use unix timestamp when sending to server, then you don't have to deal with UTC or timezones. You can convert to local time with moment.unix(unixTimestamp).format('YYYY-MM-DD HH:mm:ss')

It looks like you are using Javascript to get the time of the client, but then not passing that to the PHP. I'm not sure how your app is structured, but you could create an input tag with the type="hidden". Then using Javascript, find the element and set it's value to Date().
Here is an example: http://jsfiddle.net/43jfefuq/
Now when you submit this form with PHP, the value in the field will be the client's local time.

Display Browser specific date/time using extjs

I have a database field called CreatedDate which is a timestamp field and holds the date and time. It currently holds the GMT time.
At the moment this field is displayed as it is on the web pages.
We want to amend this to show date/time as per browser local time zone
Can you please let us know how we can do this.
Thanks in advance

If you can represent the universal time as a count of milliseconds since the 1 Jan 1970 epoch at the server then you can have the page code construct Date instances at the client using the client time zone, but starting from that UTC reference. It's the normal behavior of the Javascript Date() constructor:
var clientDate = new Date(serverUTC);
Now exactly how you get that UTC value depends on your server language. In a JSP page it'd be pretty simple:
var clientDate = new Date(<%= whatever.getTheDate().getTime() %>);
or
var clientDate = new Date(${something.theDate.time});
Once you've got the date value as a client-side Javascript Date instance, you can just update the field(s) with the string. There are no built-in Date formatting tools, but the old standard date.js might help with its ".toString()" formatter.