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date filter that adjusts for Daylight Saving

I have a date (in UTC milliseconds) that I am displaying on a page (AngularJS 1.4.0), and the users would like to see it in Eastern time, regardless of the time zone set on their computer.
I am using the date filter, and from the documentation I see
$filter('date')(date, format, timezone)
Timezone to be used for formatting. It understands UTC/GMT and the continental US time zone abbreviations, but for general use, use a time zone offset, for example, '+0430' (4 hours, 30 minutes east of the Greenwich meridian) If not specified, the timezone of the browser will be used.
Using timestamps from today, if I use something like
{{ record.Timestamp | date:'HH:mm:ss.sss':'EST' }}
it writes the time as I expect, but I think I'm going to be off by an hour when DST rolls around. I can also specify 'EDT' and it will bump the time ahead an hour, but is there a timezone abbreviation I could use that just means "Eastern", and uses the date itself to determine whether to use EST or EDT? Or am I going to need to inspect the date and create an offset myself: either -0500/EST or -0400/EDT, depending on the date? (and it looks like timezone can only be a string, not a function, so that's going to complicate my auto-detection).
It just seems unlikely that anyone would want to hard-code the display to either EST or EDT - it's much more likely that they want the time zone, adjusted for DST.

I would use Moment.js and Moment Timezone for this kind of stuff to get the right datetime. Filter in AngularJS is just to format the datetime object and show it in UI.
You can also use some ready to use directives like Angular-Moment.
It has got all sort of filtering available.

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));

Get current datetime in JS for a specific timezone

I am working on a platform that has users from all over the world, but certain settings are bound to a predefined timezone datetime.
What does this mean? If a user connects from a different timezone the app should still show him the current time in the predefined timezone regardles from where he logs in. I need to calculate this on the client side with javascript with only the timezone string.
What I would like to do is something like this:
new TimezoneDate('timezone');
and the result should be the current datetime for that timezone.
I know there are JS libs that handle this but I am asking if there is a simple JS solution without using external libs?

Depends on your definition of simple; but not really. Javascript does not have support for timezones out of the box. I would highly recommend Moment Timezone http://momentjs.com/timezone/ since it is very user friendly. I am also in the middle of a project with the same requirements and this library has made my work a lot easier.
Edit:
With the library, doing what you want is as easy as:
var timezonedDate = moment.tz('YOUR TZ STRING').toDate();

You can try:
var offset = new Date().getTimezoneOffset();
Or to get the difference in hours:
var x = new Date();
var currentTimeZoneOffsetInHours = x.getTimezoneOffset() / 60;
Mozilla Reference
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.

Convert unix timestamp with a timezone to javascript date

I have a legacy web app that stores dates as a UNIX timestamp (seconds since the epoch 1970). Usually a timestamp like this represents UTC, however these timestamps are UTC-8. It doesn't look like it ever accounts for Daylight Savings Time (DST). I could convert it on the server to UTC and send to the client, but I would like to know if there is a javascript only solution.
Example Input:
1399335987
Example Output:
"2014-05-05T16:26:27-07:00" // Pacific Daylight Time (PDT)
The client should display the date/time according to their local machine. I looked into using momentjs but I could not find how to construct a date from a number without the number being UTC already. Is this possible?

Yes, it's possible given the unix timestamps are in UTC, with Moment Timezone (http://momentjs.com/timezone/)
moment
.unix(1399335987)
.tz('MST')
.format('YYYY-MM-DDTHH:mm:ssZ');
And you get
"2014-05-05T17:26:27-07:00"
Note here I'm using MST, and you should be able to use whatever timezone you want, via Timezone Data Builder (http://momentjs.com/timezone/data/)
Actually, by default, moment parses and displays in local time.
This means, only if you're in a different timezone (offset really) and still want to get the local time in MST, it's necessary to set the timezone as MST.
Otherwise, moment.unix(1399335987).format('YYYY-MM-DDTHH:mm:ssZ') is good to go.

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.