We Keep Coding

Convert time data to local time in C#

I've read a few posts about similar subjects but nothing seems to answer this question. My database has the following information about a time
Day of the week (a number between 0-6)
Time (a number of milliseconds since midnight in the users local time)
UTC offset ( number of hours different to UTC )
DST Observed (boolean stating if DST is observed in that time zone)
This data represents opening hours. So there is a time for each day. I want to display that time in the users local time making the assumption that each day is in the future
int dayOffset = availability.Day - (int)now.DayOfWeek
if (dayOffset < 0)
dayOffset += 7;
I'm really struggling to get my head around time zones and handling when one time zone might be observing DST while another maybe DOES observe DST but hasn't yet.
My main issue at the moment is I think I need to create a DateTimeOffset object for the non-local time but I'm not sure how to do that as I don't know if DST is in effect or not.
I hope I'm making myself clear. It really is a mind-bending experience working with dates and time!

As indicated by other answers, the usual solution to handling DateTime across time zones would be to store UTC times.
However, considering that you are not referencing an absolute time at a specific date, but instead are referring to a time at an infinite number of days in a specific time zone; storing the time as an UTC time doesn't make sense anymore, since the UTC time (even if we discard the date) would be different depending on the date, due to DST.
The best way to store the time is fairly close to what you have done already.
Your problem is that the time zone information you are storing at the moment is ambiguous, as it does not refer to a specific time zone, but instead refers to properties of the time zone.
To solve this problem, simply store the time zone identifier instead of the UTC offset and DST boolean.
It is now possible for us to construct the DateTime object and convert it to any time zone by using the TimeZoneInfo class:
int dayOffset = availability.Day - (int)DateTime.Today.DayOfWeek;
if (dayOffset < 0)
{
dayOffset += 7;
}
var openingHourStart = DateTime
.SpecifyKind(DateTime.Today, DateTimeKind.Unspecified)
.AddDays(dayOffset)
.AddMilliseconds(availability.Time);
var sourceTimeZone = TimeZoneInfo.FindSystemTimeZoneById(availability.TimeZoneId);
var userTimeZone = TimeZoneInfo.Local;
var convertedOpeningHourStart = TimeZoneInfo.ConvertTime(openingHourStart,
sourceTimeZone,
userTimeZone);

Give a try to Quartz.NET.
It implements evaluation of CronExpressions, and even triggers to fire events at the given time. It can evaluate the next time an event will occur. This may help you out calculating the opening times.
Also, take a look at the cronmaker website: there you can understand the full potential of CronExpressions.
The CronExpressionDescriptor is a DotNET library for transforming CronExpressions into human readable strings.
Another library which I haven't tried yet is [HangFire].(https://www.hangfire.io/)
In this forum post you can find some discussion on how HangFire implements evaluation of RecurringJobs in local timezone with DST, which I believe is a solution for what you are looking for.

A comment to another answer made the problem a little bit more clear.
So, first and foremost, do store only UTC in your database. Really.
Now, since you are not interested in the actual dates, since you are storing working schedules that repeat weekly, the date only becomes relevant once you want to present your times - and when you put them in your database.
So let's first see how you get your times into your database correctly. I'm assuming a user will enter times in their own locale.
Make sure you first create a (localised) DateTime consisting of the current date and the given time (from the user), and transform that to a UTC DateTime (you can keep the current date, it doesn't matter):
var utcDateTime = DateTime.Now.Date
.AddHours(userHours)
.AddMinutes(userMinutes)
.ToUniversalTime();
Now when you are presenting these times to the user, simply go the other way:
var userDateTime = DateTime.Now.Date
.AddHours(utcDateTime.Hour)
.AddMinutes(utcDateTime.Minute)
.ToLocalTime();
And then you can use the userDateTime.Hour and .Minute for display purposes.

You should be leveraging DateTime.ToLocalTime() and TimeZoneInfo.ConvertTimeToUtc() in C# - see https://msdn.microsoft.com/en-us/library/system.datetime.tolocaltime(v=vs.110).aspx.
If you want to store only times that you're open from Monday to Sunday, fine. Have a simple data table to describe only the time for each day (0 = Sunday through 7 = Saturday -- this is .Net's DayOfWeek enumeration). Your lookup table might look like:
0 null
1 08:00:00
2 08:00:00
3 08:00:00
4 08:30:00
5 08:30:00
6 10:30:00
(Use whatever data type works for you--SQL Server 2008+ has a TIME data type, for example. Null can be used for Closed on that day--i.e., no open time.)
When it comes time to display YOUR time to any other user, use must create your UTC time on-the-fly at the moment you are displaying information to the local user.
Conyc provided one approach. My approach uses simple date/time strings. To use my approach, just store time values per day in your database. Then you can look up the open time for any given day. To express that time for another user in any locale, use this code to convert your time to UTC (you can substitute the "08:00:00 AM" string value with a string variable that you populated after looking up the open time in your database):
var StoreOpenTimeInUtc = TimeZoneInfo.ConvertTimeToUtc(Convert.ToDateTime("08:00:00 AM"));
To look up the open time in your database for a particular day in the future, you will need to concatenate the date to your time value, like this:
var StoreOpenTimeInUtc = TimeZoneInfo.ConvertTimeToUtc(Convert.ToDateTime("04/28/2018 08:00:00 AM"));
Once you have an accurate StoreOpenTimeInUtc variable, you can use that as the UTC value on someone else's machine who is anywhere else on planet earth. To convert that UTC value to their local time, use the .NET ToLocalTime() method:
var OpenTimeForLocalUser = StoreOpenTimeInUtc.ToLocalTime();
Note that this approach requires you to store only the open times as shown above. You don't have to worry about dates, local offsets from UTC, or anything else. Just leverage ConvertTimeToUtc() and ToLocalTime() as shown.

UTC to local time conversion for previously saved datetimes if rules for timezone change

I'm storing a product in db. All dates (sql server datetime) are UTC and along with the dates I store the time zone id for that product. User enters the dates when product is available "from" and "until" in the listing.
So I do something like:
// Convert user's datetime to UTC
var userEnteredDateTime = DateTime.Parse("11/11/2014 9:00:00");
// TimeZoneInfo id will be stored along with the UTC datetime
var tz = TimeZoneInfo.FindSystemTimeZoneById("FLE Standard Time");
// following produces: 9/11/2014 7:00:00 AM (winter time - 1h back)
var utcDateTime = TimeZoneInfo.ConvertTimeToUtc(userEnteredDateTime, tz);
and save the record. Let's assume user did this on the 1st of August, while his time zone offset to UTC is still +03:00, nevertheless the saved date for the future listing has the correct +02:00 value because conversion took into consideration the "winter" time for that period.
Question is what datetime value will I get if I will attempt to convert that product's "from" and "until" date to product's local time zone on 11/11/2014 if, for example, due to some new rules the transition to winter time was abandoned, thus the time zone is still +03:00 instead of +02:00?
// Convert back
var userLocalTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, tz);
will I get 10AM or correct 9AM because OS/.NET patch will handle this?
Thank you!
P.S.: TimeZoneInfo has ToSerializedString() method, if I rather store this value instead of timezone id, will this guarantee that via UTC datetime + serialized timezoneinfo I will always be able to convert to the user's original datetime input?

In the scenario you describe, you would get 10:00 AM. The time zone conversion function would not have any idea that the value was originally entered as 9:00 AM, because you only saved the UTC time of 7:00 AM.
This illustrates one of the cases where the advice "always store UTC" is flawed. When you're working with future events, it doesn't always work. The problem is that governments change their mind about time zones often. Sometimes they give reasonable notice (ex. United States, 2007) but sometimes they don't (ex. Egypt, 2014).
When you made the original conversion from local time to UTC, you intentionally decided to trust that the time zone rules would not change. In other words, you decided that you would assign the event to the universal timeline based solely on the time zone rules as you knew them at that time.
The way to avoid this is simple: Future events should be scheduled in local time. Now, I don't mean "local to your computer", but rather "local to the user", so you will need to know the user's time zone, and you should also store the ID of the time zone somewhere.
You'll also need to decide what you want to do if the event falls into the spring-forward or fall-back transition for daylight saving time. This is especially important for recurrence patterns.
Ultimately though, you'll need to figure out when to run the event. Or in your case, you'll need to decide if the event has passed or not. There are a few different ways you can accomplish this:
Option 1
You can calculate the corresponding UTC value for each local time and keep it in a separate field.
On some cycle (daily, weekly, etc) you can recalculate upcoming UTC values from their local values and your current understanding of the time zone rules. Or, if you apply time zone updates manually, you can choose to recalculate everything at that time.
Option 2
You can store the values as a DateTimeOffset type instead of a DateTime. It will contain the original local time, and the offset that you calculated based on the time zone rules as you knew them at time of entry.
DateTimeOffset values can easily be coerced back to UTC, so they tend to work very well for this. You can read more in DateTime vs DateTimeOffset.
Just like in option 1, you would revisit the values periodically or after time zone data updates, and adjust the offsets to align with the new time zone data.
This is what I usually recommend, especially if you're using a database that has support for DateTimeOffset types, such as SQL Server or RavenDB.
Option 3
You can store the values as a local DateTime.
When querying, you would calculate the current time in the target time zone and compare against that value.
DateTime now = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, targetTZ);
bool passed = now >= eventTime;
The down side of this option is that you may have to make lots queries if you have events in lots of different time zones.
You may also have issues with values close to the fall-back DST transition, so be careful if you use this approach.
I recommend against the idea of serializing the time zone itself. If the time zone has changed, then it has changed. Pretending that it hasn't isn't a good workaround.

DateTime.Now and Culture/Timezone specific

Our application was designed to handle user from different Geographic location.
We are unable to detect what is the current end user local time and
time zone operate on it. They select different culture like sv-se,
en-us, ta-In even they access from Europe/London timezone..
We hosted it in a hosting server in US, application users are from Norway/Denmark/Sweden/UK/USA/India
The problem is we used DateTime.Now to store the record created/updated date, etc.
Since the Server runs in USA all user data are saved as US time :(
After researching in SO, we decided to stored all history dates in DB as DateTime.UtcNow
PROBLEM:
There is a record created on 29 Dec 2013, 3:15 P.M Swedish time.
public ActionResult Save(BookingViewModel model)
{
Booking booking = new Booking();
booking.BookingDateTime = model.BookingDateTime; //10 Jan 2014 2:00 P.M
booking.Name = model.Name;
booking.CurrentUserId = (User)Session["currentUser"].UserId;
//USA Server runs in Pacific Time Zone, UTC-08:00
booking.CreatedDateTime = DateTime.UtcNow; //29 Dec 2013, 6:15 A.M
BookingRepository.Save(booking);
return View("Index");
}
We want to show the same history time to the user who logged in in India/Sweden/USA.
As of now we are using current culture user logged in and choose the timezone from a config file and using for conversion with TimeZoneInfo class
<appSettings>
<add key="sv-se" value="W. Europe Standard Time" />
<add key="ta-IN" value="India Standard Time" />
</appSettings>
private DateTime ConvertUTCBasedOnCulture(DateTime utcTime)
{
//utcTime is 29 Dec 2013, 6:15 A.M
string TimezoneId =
System.Configuration.ConfigurationManager.AppSettings
[System.Threading.Thread.CurrentThread.CurrentCulture.Name];
// if the user changes culture from sv-se to ta-IN, different date is shown
TimeZoneInfo tZone = TimeZoneInfo.FindSystemTimeZoneById(TimezoneId);
return TimeZoneInfo.ConvertTimeFromUtc(utcTime, tZone);
}
public ActionResult ViewHistory()
{
List<Booking> bookings = new List<Booking>();
bookings=BookingRepository.GetBookingHistory();
List<BookingViewModel> viewModel = new List<BookingViewModel>();
foreach (Booking b in bookings)
{
BookingViewModel model = new BookingViewModel();
model.CreatedTime = ConvertUTCBasedOnCulture(b.CreatedDateTime);
viewModel.Add(model);
}
return View(viewModel);
}
View Code
#Model.CreatedTime.ToString("dd-MMM-yyyy - HH':'mm")
NOTE: The user can change the culture/language before they login. Its a localization based application, running in US server.
I have seen NODATIME, but I could not understand how it can help with multi culture web application hosted in different location.
Question
How can I show a same record creation date 29 Dec 2013, 3:15 P.M for the users logged in INDIA/USA/Anywhere`?
As of now my logic in ConvertUTCBasedOnCulture is based user logged in culture. This should be irrespective of culture, since user can login using any culture from India/USA
DATABASE COLUMN
CreatedTime: SMALLDATETIME
UPDATE: ATTEMPTED SOLUTION:
DATABASE COLUMN TYPE: DATETIMEOFFSET
UI
Finally I am sending the current user's local time using the below Momento.js code in each request
$.ajaxSetup({
beforeSend: function (jqXHR, settings) {
try {
//moment.format gives current user date like 2014-01-04T18:27:59+01:00
jqXHR.setRequestHeader('BrowserLocalTime', moment().format());
}
catch (e) {
}
}
});
APPLICATION
public static DateTimeOffset GetCurrentUserLocalTime()
{
try
{
return
DateTimeOffset.Parse(HttpContext.Current.Request.Headers["BrowserLocalTime"]);
}
catch
{
return DateTimeOffset.Now;
}
}
then called in
model.AddedDateTime = WebAppHelper.GetCurrentUserLocalTime();
In View
#Model.AddedDateTime.Value.LocalDateTime.ToString("dd-MMM-yyyy - HH':'mm")
In view it shows the local time to user, however I want to see like dd-MMM-yyyy CET/PST (2 hours ago).
This 2 hours ago should calculate from end user's local time. Exactly same as stack overflow question created/edited time with Timezone display and local user calculation.
Example: answered Jan 25 '13 at 17:49 CST (6 hours/days/month ago) So the other viewing from USA/INDIA user can really understand this record was created exactly 6 hours from INDIA/USA current time
Almost I think I achieved everything, except the display format & calculation. How can i do this?

It sounds like you need to store a DateTimeOffset instead of a DateTime. You could just store the local DateTime to the user creating the value, but that means you can't perform any ordering operations etc. You can't just use DateTime.UtcNow, as that won't store anything to indicate the local date/time of the user when the record was created.
Alternatively, you could store an instant in time along with the user's time zone - that's harder to achieve, but would give you more information as then you'd be able to say things like "What is the user's local time one hour later?"
The hosting of the server should be irrelevant - you should never use the server's time zone. However, you will need to know the appropriate UTC offset (or time zone) for the user. This cannot be done based on just the culture - you'll want to use Javascript on the user's machine to determine the UTC offset at the time you're interested in (not necessarily "now").
Once you've worked out how to store the value, retrieving it is simple - if you've already stored the UTC instant and an offset, you just apply that offset and you'll get back to the original user's local time. You haven't said how you're converting values to text, but it should just drop out simply - just format the value, and you should get the original local time.
If you decide to use Noda Time, you'd just use OffsetDateTime instead of DateTimeOffset.

Standard approach is to always store any time data as UTC if particular moment in time is important. That time is not impacted by time zone changes and cultures.
Most common approach to showing time with time zone is to store time as UTC and convert to current user's culture/time zone combination when you display the value. This approach only requires single date time filed in the storage.
Note that for Web cases (like ASP.Net) you may need to figure out user's culture/time zone first and send it to server (as this information is not necessary available on GET requests) or do formatting of time in the browser.
Depending what "show the same history time" you may need to store additional information like current culture and/or current offset. If you need to show time exactly as original user seen it you may also save string representation (because formats/translations can change later and value will look different, also it is unusual).
Note: culture and time zone are not tied together, so you'll need to decide how you need to handle cases like IN-IN culture in US PST time zone.

I'm a little confused by the phrasing of your question, but it appears that you would like to determine the time zone of your user.
Have you tried asking them? Many applications have the user pick their time zone in user settings.
You could pick from a drop-down list, or a pair of lists (country, then time zone within the country), or from a map-based time zone picker control.
You could take a guess and use that as the default unless your user changes it.
If you go down that route, you will need to be able to use IANA/Olson time zones, which is where Noda Time comes into play. You can access them from DateTimeZoneProviders.Tzdb.
The hosting location is irrelevant if you are using UTC. That's a good thing.
Also, if you're using Noda Time, then you probably should use SystemClock.Instance.Now instead of DateTime.UtcNow.
See also here and here.
Also - an alternative solution would be just to pass the UTC time to the browser and load it into a JavaScript Date object. The browser can convert that to the user's local time. You could also use a library like moment.js to make this easier.
Update
Regarding your approach of mapping culture codes to time zones:
<appSettings>
<add key="sv-se" value="W. Europe Standard Time" />
<add key="ta-IN" value="India Standard Time" />
</appSettings>
That will not work, for several reasons:
Many people use a different culture setting on their computer than the area that they are physically in. For example, I might be an a US-English speaker living in Germany, my culture code is likely still en-US, not de-DE.
A culture code containing a country is used to distinguish between dialects of a language. When you see es-MX, that means "Spanish, as spoken in Mexico". It does not mean that the user is actually in Mexico. It just means that user speaks that dialect of Spanish, as compared to es-ES which means "Spanish, as spoken in Spain".
Even if the country portion of the culture code could be reliable, there are many countries that have multiple time zones! For example, what would you put in your mapping list for en-US? You can't just assume that we are all on Eastern Standard Time.
Now, I've explained why your current approach won't work, I strongly suggest you take my original advice. Very simply:
Determine the time zone of the user, preferably by asking them, perhaps with some assistance by one of the utilities I linked to above.
You're storing UTC, so just convert to that time zone for display.
Using Microsoft Time Zones
TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("W. Europe Standard Time");
DateTime localDatetime = TimeZoneInfo.ConvertTimeFromUtc(yourUTCDateTime, tz);
Using IANA Time Zones and Noda Time
DateTimeZone tz = DateTimeZoneProviders.Tzdb["Europe/Stockholm"];
Instant theInstant = Instant.FromDateTimeUtc(yourUTCDateTime);
LocalDateTime localDateTime = theInstant.InZone(tz);

We faced a similar problem with an application I worked on recently. During development every one was in the same time zone and the issue wasn't noticed. And in any case there was a lot of legacy code that would have been a pain to change not to mention converting all the date time info that was already in the DB. So changing to DateTimeOffset was not an option. But we managed to get it all consistent by converting from server time to user time on the way out and converting from user time to server time on the way in. It was also important to do this with any date time comparisons that were a boundary. So if a user expected some thing to expire midnight their time then we would convert that time to server time and do all comparisons in server time. This sounds like a lot of work but was much less work then converting the entire application and DB to use DateTimeOffsets.
Hear is a thread that looks like has some good solutions to the time zone issue.
Determine a User's Timezone

If you want to show consistent date/time history to the user, regardless of the locale they are viewing the history from, then:
During Save, store not only UTC "creation" date/time, but also the detected locale
Use stored saved from locale to compute original date/time and emit a string to display (i.e. do not use current user locale when you're diplaying it)
If you don't have the ability to amend your storage, then perhaps you can change your submit to send the "current client time", store it literally (do not convert to UTC) and then display literally (do not convert to detected culture)
But as I say in my comment under your question, I am not certain I got your requirements right.

Converting any date time from server time to user time (dealing with standard and daylight time)

I am working on a web app using .NET MVC 3 and SQL server 2008 (r2).
Anyway, I have a date time object and I want to convert it to user time. It is fairly trivial to convert right now to user time; I have some java script that will get me the users offset from UTC. And I know my date times offset to UTC.
What I realized yesterday, is that my user's offsets will change, if they live in a pesky area of the world. And that would cause old dates and time to be wrong by some amount of time.
Now I know C# has some utilities to convert into a time zones time, but do they really handle all of the intricacies of daylights savings?
For example. If I have a time, 10-10-2001 8:00:00 how do I get that into pacific time? Or if i have 6-6-2004, how do i get that into pacific time?
Since the dates change for the roll overs, it is pretty complex, and I would like a general solution that does not require maintenance to set up date ranges for time zones.
I know this is a classic problem in CS, but I cannot find something that really answers my question 100%. From what I have seen: C# uses the current year's daylights savings dates to change every years date times. Which could cause some mistakes.
Thank you so much for the help.

Now I know C# has some utilities to convert into a time zones time, but do they really handle all of the intricacies of daylights savings?
TimeZoneInfo does, yes1. (It's part of the .NET framework, not part of C# - C# is just the language you happen to be using.) However, I don't think that's what you really want to do.
Why are you storing the DateTime in the server's time zone anyway? It would be more sensible to store it in UTC, in most cases. Aside from anything else, if your server is in a time zone which observes daylight saving time, you will end up with ambiguity for one hour per year, when the clock goes back. (The same local time occurs twice.)
Once you've stored it as UTC, you should give it to your Javascript client as UTC too. While you say that you have "some java script that will get me the users offset from UTC" - that will depend on the exact instant in time. For example, as I'm in the UK, my offset is sometimes 0 and sometimes +1 hour. If you pass the UTC back to the client, that can work out the local time from that UTC time. Your server can't, unless you can get an accurate representation of the time zone from the client to the server, which is generally a tricky thing to do.
From what I have seen: C# uses the current year's daylights savings dates to change every years date times.
Again, C# itself isn't relevant here. It's not clear which part of the .NET framework you mean - TimeZone? TimeZoneInfo? DateTime? TimeZoneInfo has historical data, but only if you're on an operating system version which supports it.
1 Well, as far as you're likely to care. It doesn't have as much historical data as TZDB, and it has some very odd representations for Russia and Namibia, but it does generally have the idea of rules changing.

I have found the following rules to be the best solution:
At server-side use UTC. No need to deal with Time Zones at all. That means that if you store dates in database, use UTC dates, if you generate dates in server side use generally UTC (e.g: var nowDate =DateTime.UtcNow). The server dates should be location agnostic and that way there is no confusion.
At client-side use UTC as well when dealing with raw data, but display the dates in the UI converted to the browser's local time (which will be whatever time is configured in the operating system).
The good news is that this is already a built in capability in browsers! we just need to make it easier for them to handle it.
This means that if we have a javascript client code POSTing/PUTing/PATCHing data to a server and GETting data from the server using JSON, the JSON should use the ISO 8601 (natively supported) in UTC (ended in Z) as if you were doing:
var dateNow = new Date(); //Wed May 11 2016 13:06:21 GMT+1200 (New Zealand Standard Time)
var dateNowIso = dateNow.toISOString(); //"2016-05-11T01:06:21.147Z"
And then, simply rely on the browser capability to convert automatically any UTC date to the current local time and vice versa. The browser's javascript engine will be able to display in the UI the correct local time as long as the format is a Date object.
NOTE on how to implement it for a .NET server & Javascript client scenario: For example, I am using AngularJS (with restangular) in my client javascript code and MVC 6 (ASP.NET Core 1.0) in server side.
The JSON that comes from the server might contain date properties, but in JSON the type is a string such as:
{
"myDateField":"2016-05-11T05:00:00Z"
}
In order to rely on the browser's capability to handle properly UTC times and convert them to local browser's time I need to parse this UTC date string into a real javascript Date object, so I use a regular expression to match any text value that looks like a UTC date in ISO 8601 format (in my case this code is in a restangular response interceptor but it could be anywhere):
const regexIso8601 = /((((\d{4})(-((0[1-9])|(1[012])))(-((0[1-9])|([12]\d)|(3[01]))))(T((([01]\d)|(2[0123]))((:([012345]\d))((:([012345]\d))(\.(\d+))?)?)?)(Z|([\+\-](([01]\d)|(2[0123]))(:([012345]\d))?)))?)|(((\d{4})((0[1-9])|(1[012]))((0[1-9])|([12]\d)|(3[01])))(T((([01]\d)|(2[0123]))(([012345]\d)(([012345]\d)(\d+)?)?)?)(Z|([\+\-](([01]\d)|(2[0123]))([012345]\d)?)))?))/;
function convertDateStringsToDates(input: any): void {
// Ignore things that aren't objects.
if (typeof input !== "object") {
return input;
}
for (var key in input) {
if (!input.hasOwnProperty(key)) {
continue;
}
let value = input[key];
let match: RegExpMatchArray;
// Check for string properties which look like dates.
if (angular.isArray(value)) {
angular.forEach(value, (val, key) => {
convertDateStringsToDates(val);
});
} else if (angular.isString(value) && (match = value.match(regexIso8601))) {
let milliseconds = Date.parse(match[0]);
if (!isNaN(milliseconds)) {
input[key] = new Date(milliseconds);
}
} else if (angular.isObject("object")) {
// Recurse into object
convertDateStringsToDates(value);
}
}
}
That way I can use my javascript object with proper Date objects.
In the server side (either MVC6 or WebAPI2..) I have an instruction to tell my JSON parser to ALWAYS use UTC formats when parsing Dates, otherwise it wouldn't add the Z character at the end, which is very important. Example for MVC6:
services.AddMvc().AddJsonOptions(opt => {
// json dates always in javascript date format with UTC e.g: "2014-01-01T23:28:56.782Z"
opts.SerializerSettings.DateTimeZoneHandling = Newtonsoft.Json.DateTimeZoneHandling.Utc;
});
Inspired by this article
UPDATE: There are scenarios where it might be interesting to store data in local time as stated in the comments below.

TimeZoneInfo and country codes

I'm looking to implement a function with a signature something like the following:
bool IsTimeZoneValid(string countryCode, DateTime localTime);
The intention is to determine whether the country has a time zone in which the local time would be valid, given that we know the current UTC time. Let's say, for the sake of argument, that "valid" means that when converted to UTC the time is +/- 30 minutes from what we believe the time is.
For example, say it is currently 03/08/2009 18:25:00 UTC then given the following method call for Australia, it should return true as this is a valid time in the "Eastern Standard Time" zone:
IsTimeZoneValid("AU", DateTime.Parse("04/08/2009 03:25:00"));
However the following call for France should fail as this is not a valid time in France's time zone.
IsTimeZoneValid("FR", DateTime.Parse("04/08/2009 03:25:00"));
This needs to be accurate, and take into account daylight saving time etc.
.NET 3.5 includes the new TimeZoneInfo class which can do a lot of the conversion for me if I know what time zones exist in a specific country, but I can't seem to find any built-in lookup for this. Am I missing something, or am I going to have to manually create a table of country to time zone mappings?
To reiterate, my question is: Given a country code, how can I get a list of time zones. Alternatively, is there another way to approach this that I've missed?

By default windows only adds Time Zone info for you local time zone, which may be the cause of the problem.
This class works only for the local time zone and any predefined time zones. If you want to use this for other times zones, you must have the registry settings added to the machine for all time zones needed or create custom time zone info using CreateCustomTimeZone.
http://msdn.microsoft.com/en-us/library/system.timezoneinfo.aspx?ppud=4
http://msdn.microsoft.com/en-us/library/bb384268.aspx