How does the conversion to UTC from the standard DateTime format work?
More specifically, if I create a DateTime object in one time zone and then switch to another time zone and run ToUniversalTime() on it, how does it know the conversion was done correctly and that the time is still accurately represented?
There is no implicit timezone attached to a DateTime object. If you run ToUniversalTime() on it, it uses the timezone of the context that the code is running in.
For example, if I create a DateTime from the epoch of 1/1/1970, it gives me the same DateTime object no matter where in the world I am.
If I run ToUniversalTime() on it when I'm running the code in Greenwich, then I get the same time. If I do it while I live in Vancouver, then I get an offset DateTime object of -8 hours.
This is why it's important to store time related information in your database as UTC times when you need to do any kind of date conversion or localization. Consider if your codebase got moved to a server facility in another timezone ;)
Edit: note from Joel's answer - DateTime objects by default are typed as DateTimeKind.Local. If you parse a date and set it as DateTimeKind.Utc, then ToUniversalTime() performs no conversion.
And here's an article on "Best Practices Coding with Date Times", and an article on Converting DateTimes with .Net.
Firstly, it checks whether the Kind of the DateTime is known to be UTC already. If so, it returns the same value.
Otherwise, it's assumed to be a local time - that's local to the computer it's running on, and in particular in the time zone that the computer was using when some private property was first lazily initialized. That means if you change the time zone after your application was started, there's a good chance it will still be using the old one.
The time zone contains enough information to convert a local time to a UTC time or vice versa, although there are times that that's ambiguous or invalid. (There are local times which occur twice, and local times which never occur due to daylight saving time.) The rules for handling these cases are specified in the documentation:
If the date and time instance value is
an ambiguous time, this method assumes
that it is a standard time. (An
ambiguous time is one that can map
either to a standard time or to a
daylight saving time in the local time
zone) If the date and time instance
value is an invalid time, this method
simply subtracts the local time from
the local time zone's UTC offset to
return UTC. (An invalid time is one
that does not exist because of the
application of daylight saving time
adjustment rules.)
The returned value will have a Kind of DateTimeKind.Utc, so if you call ToUniveralTime on that it won't apply the offset again. (This is a vast improvement over .NET 1.1!)
If you want a non-local time zone, you should use TimeZoneInfo which was introduced in .NET 3.5 (there are hacky solutions for earlier versions, but they're not nice). To represent an instant in time, you should consider using DateTimeOffset which was introduced in .NET 2.0SP1, .NET3.0SP1 and .NET 3.5. However, that still doesn't have an actual time zone associated with it - just an offset from UTC. That means you don't know what local time will be one hour later, for example - the DST rules can vary between time zones which happened to use the same offset for that particular instant. TimeZoneInfo is designed to take historical and future rules into account, as opposed to TimeZone which is somewhat simplistic.
Basically the support in .NET 3.5 is a lot better than it was, but still leaves something to be desired for proper calendar arithmetic. Anyone fancy porting Joda Time to .NET? ;)
What #womp said, with the addition that it checks the DateTime's Kind property to see if it might already be a UTC date.
DateTime.ToUniversalTime removes the timezone offset of the local timezone to normalize a DateTime to UTC. If you then use DateTime.ToLocalTime on the normalized value in another timezone, the timezone offset of that timezone will be added to the normalized value for correct representation in that timezone.
Related
I have a solution in Azure in Ireland. I live in Denmark.
Would it be correct, to save all my times and dates as universal = DateTime.Now.ToUniversalTime() in my database in Ireland?
When I need the danish time, I cannot: universal.ToLocalTime() because I’m still on the server in Ireland. Instead I could write:
danish = TimeZoneInfo.ConvertTimeFromUtc(universal, TimeZoneInfo.FindSystemTimeZoneById("Romance Standard Time"));
But I think it’s a long way.
Alternative 1: I convert date to danish, before I save in my database in Ireland.
Alternative 2: There are one timezone between Denmark and Ireland, so I could: danish = dateTime.AddHours(1), but perhaps it could give errors with the summer and winther time.
A few things:
There's nothing wrong with writing DateTime.Now.ToUniversalTime(), but understand that under the hood that is fetching the current UTC time, converting to local time, then converting back to UTC. Thus, it's both shorter and more efficient to simply call DateTime.UtcNow to get the UTC time directly without conversions.
The server on which your code is running is irrelevant (or should be). Additionally, all Azure instances have their local time zone set to UTC anyway, so you will not see any effects from Ireland's local time zone.
Yes, "Romance Standard Time" is the correct Windows identifier for the local time in Denmark. If you are running on a non-Windows platform, you should use "Europe/Copenhagen" instead. If you are writing for multi-platform usage, then use "Europe/Copenhagen" with my TimeZoneConverter library.
If you just wanted to write shorter code, you might be able to do the following:
DateTime danish = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(universal, "Romance Standard Time");
I say might, because this will only be correct if the DateTime you've got in your universal variable has its Kind property set to DateTimeKind.Utc. If you've set that previously using DateTime.SpecifyKind, then this will work. On the other hand, if you have just retrieved this value from a database, then by default the Kind will by DateTimeKind.Unspecified. The key difference being that ConvertTimeFromUtc treats DateTimeKind.Unspecified as if it were DateTimeKind.Utc, while ConvertTimeBySystemTimeZoneId treats DateTimeKind.Unspecified as if it were DateTimeKind.Local.
In other words, if you don't set the Kind explicitly, it would convert from local time to Danish time. However, since Azure runs its local time zone as UTC, you'd get the same result. Just you might get different results when running elsewhere.
You might consider using DateTimeOffset instead of DateTime. This usually results in clearer code and less errors. (DateTimeKind is not used with DateTimeOffset.) Of course it depends on what exactly you're using this data for, but often it's a good idea.
About your proposed alternatives, I suggest neither. Your original code is better. My thoughts on each:
Alternative 1: I convert date to danish, before I save in my database in Ireland.
It depends on what you're representing.
If this is a unique point in time (the time something happened, usually in the past) - then you should keep it as UTC. That allows conversion to any time zone correctly.
On the other hand, if you are representing the time something is scheduled to happen (in the future), then most often it's the local time that matters most. This is especially important for recurring events in time zones with DST, or events in time zones that might be volatile (where the government makes frequent short-notice changes).
Alternative 2: There are one timezone between Denmark and Ireland, so I could: danish = dateTime.AddHours(1), but perhaps it could give errors with the summer and winter time.
You should not add or subtract time to adjust time zones. After all - you're not talking about a different point in time an hour in the future or an hour in the past.
Also, Denmark and Ireland may be similar today, but that doesn't mean they have always been that way, and it doesn't mean they will necessarily stay that way in the future. Ireland has a different time zone identifiers ("GMT Standard Time" on Windows, "Europe/Dublin" on other platforms).
Use server Utc time to save record in database in order to avoid any time differences when converting datetime from and then converting it back. Once the record use Utc datetime then you can easily convert it to any time zone, so for example: system requires to convert datetime to Denmark local time, and in future it might need to show in Germany datetime
DateTime timeUtc = DateTime.UtcNow;
TimeZoneInfo cstZone = TimeZoneInfo.FindSystemTimeZoneById("Romance Standard Time");
DateTime cstTime = TimeZoneInfo.ConvertTimeFromUtc(timeUtc, cstZone);
We have an application related to aviation, and specifically flights.
Times have to be stored in local, so I chose to use UTC Time + an offset, but then now I realize it is a bad choice:
By storing the timezone as an offset, I lose track of the original timezone and this has implications when dealing with daylight savings.
For example I can store a time in Alpine, UT as UTC time and a -6 offset and also a time in Phoenix, AZ as UTC time and a -6 offset.
But when daylight saving comes, the time will change in Alpine, but not in Phoenix.
So, I need to store the proper timezone and I have seen that there are also different lists with a different syntax, so I am assuming there are different standards.
In C#, what would be the best option to store a local time with the local time zone to make it work with daylight saving changes?
From the discussion in the question's comments, I understand that you are working with flight time schedules - that is, the time a future flight is intended to depart. This is indeed a case where the local time is more important than the UTC time.
Since you have the local time and location of departure (ex: 5:00 PM in Salt Lake City), then you should be storing in your database of scheduled departure times two values:
17:00 - The relevant local time of the departure
SLC - The location where the time is relevant
If this is a specific occurrence of this flight, then you should store the date as well:
2018-06-01T17:00 - The specific relevant local time of the departure
SLC - The location where the local time is relevent
These are the details that are contextually relevant to your business use case. Do not lose sight of them by converting them to UTC.
That said, you might consider storing them as a DateTimeOffset (2018-06-01T17:00-06:00), which makes converting to UTC trivial for a given instance. However there are two problems with this approach:
It cannot work with recurrences, because the offset may change.
Even for a single instance, the offset might change - if the government controlling the time zone decides to change their standard offset or daylight saving time rules before that occurrence takes effect. If you do take a DateTimeOffset approach, or a UTC-based approach, you must be prepared to recalculate future events in the face of such changes. (For more on this, see my blog articles: On the Timing of Time Zone Changes and Time Zone Chaos Inevitable in Egypt. There are countless other examples.)
With regards to the location - because you are working with data that is contextually applicable to the airline industry, I recommend using IATA airport codes like the SLC that I showed above. In other contexts, one might store an IANA time zones identifier like America/Denver, or a Windows time zone identifier like Mountain Standard Time.
You may find my "Airport Time Zones" gist (code and output table) useful for working with IATA airport codes. You'll have to decide how that data will flow through your system. If you are running on Windows and want to use the TimeZoneInfo class to convert times to different time zones, then use the Windows time zone IDs shown there. If you want to use the IANA time zone IDs instead, consider using Noda Time, or you can use my TimeZoneConverter library. There are several different options here, so explore them all carefully and pick ones that make sense to you.
Noda Time would be a great choice, IMHO. Not only would you get excellent time zone support, but you'd also be able to use types like LocalTime or LocalDateTime which align well with the scenarios described.
As I wrote in my comment, do not store local dates. Instead, store datetime values as UTC, and convert to local datetime when you need to display it.
You can use the ConvertTimeFromUtc method of the TimeZoneInfo class for that.
This means you will have to also keep a list of locations and whatever TimeZoneInfo they are associated in - For example,
Jerusalem would be associated with Israel Standard Time,
Rome with W. Europe Standard Time,
Hawaii with Hawaiian Standard Time
and so on. (I'll bet you can find such a list online somewhere.)
Please note that the ConvertTimeFromUtc method handles the daylight savings problem for you as well.
Then you can do something like this to get the local time by location:
DateTime GetLocalDateByCityName(DateTime utc, string cityName)
{
var timeZoneInfoId = GetTimeZoneInfoIdByCityName(string cityName);
return TimeZoneInfo.ConvertTimeFromUtc(utc, TimeZoneInfo.FindSystemTimeZoneById(timeZoneInfoId);
}
And of course, in GetTimeZoneInfoIdByCityName you get the TimeZoneInfoId for the specific city.
You should use TimeZoneInfo.ConvertTime to convert DateTime between time zones if you need to handle Daylight Savings.
TimeZoneInfo.ConvertTime(myDateTime, timeZone);
TimeZoneInfo.ConvertTime(myDateTime, fromTimeZone, toTimeZone);
The back-end is C# and SQL Server.
I save logging activity date times from different time zones in UTC in my DB. I need to populate a globally distributed front end so it localizes the logged UTC time based on where the front end is located, which will be known e.g. PT, ET. It also needs to handle daylight savings time which is where I'm running into difficulty.
As the UTC offset changes based on the time of the year because of daylight savings time, is there a way for me to find out what the UTC offset is in C# given that I know the date/time and the time zone.
is there a way for me to find out what the UTC offset is in C# given that I know the date/time and the time zone
Technically you shouldn't need to find out this information, the underlying system clock/calendar/what-have-you should take care of it. (I'm sure anybody who has worked on date-time logic will attest to the fact that it is non-trivial to say the least.)
Ideally, any time you need to convert a time zone, you have three things:
A DateTime object representing the value to be converted.
A TimeZoneInfo object representing the known current format of the DateTime object (in this case UTC).
A TimeZoneInfo object representing the target time zone.
Given those things, any time you want to display the DateTime to a localized user, you'd use something like TimeZoneInfo.ConvertTime to perform the conversion. This should ideally be done as close to the UI as possible so that backing logic is always consistently performed in UTC.
I'm working on an ASP.NET/C# application, which supports time zones.
First let me explain the flow of the application, I'm storing an object for purchase order. So it has the datetime field in it.
I'm storing that datetime as UTC in Database, and binding it in grid (according to the client's timezone).
In the Page_Init method of the first page, I had used a javascript code which will detect the client's timezone, and return appropriate offset.
In the Page_Load method, I'm getting that javascript return value (offset) and comparing it with the offset of each zone in TimeZoneInfo.GetSystemTimeZones().
When comparing the offset, I'm getting a TimeZoneInfo object (for example) "(UTC-08:00) Baja California", "(UTC +05:30) Chennai,Kolkata"
Using that particular TimeZoneInfo object, I'm converting the UTC datetime (which is stored in DB) to the client's timezone.
As by the above flow, the application works fine for some timezones.
The problem is, if when I change timezone of client machine to (UTC -8:00), the client machine shows the timezone name as "(UTC-08:00) Pacific Time (US & Canada)" but in application the timezone differs from the client system it shows as "(UTC-08:00) Baja California".
And importantly DST changes are not reflecting when I convert the UTC to Local. TimeZoneInfo.ConvertTimeFromUtc(DateTime, clientTimezone);
Note: I'm not storing Timezone information of client in database or anywhere. So everytime if a user enters the application, the application will recognize the timezone and it will react according to it.
My question is:
Whether the TimeZoneInfo class can work automatically work according to the adjusment rule, when we convert from UTC to Local?
Do we have to detect the DST for particular datetime using the method TimeZoneInfoObject.IsDaylightSavingTime(DateTime) and do conversion?
Is there any other classes in .Net which can sync with windows timezones?
A few things you should understand:
A time zone is not the same as a time zone offset. One cannot just take the number -8 and assume that the time zone should be Pacific time.
Offsets can change within a single time zone. For example, Pacific Time usually uses -8, but switches to -7 when daylight saving time is in effect.
The offsets in the DisplayName property of a TimeZoneInfo are only the standard offset. They match with the BaseOffset property. They do not change to reflect the current offset.
Time zone detection in JavaScript is imperfect. There are only three approaches:
Using the getTimezoneOffset function of the Date class, which should return you the offset of the date it was called on. For example new Date().getTimezoneOffset() gives you the current offset. With this approach, you should also be aware that there is a bug in the ES5 spec that can cause the wrong offset to sometimes be returned when called on older dates.
Using a library such as jsTimezoneDetect, which makes several calls to the getTimezoneOffset to attempt to guess at an IANA time zone identifier. The guess is suitable to set a default time zone when a list of time zones is presented to the user. It is just a guess, and could be wrong. If you want to use it on the back end with .NET, you'll need Noda Time, since TimeZoneInfo doesn't currently support IANA time zones. (You can optionally convert to Windows time zones if desired).
Some newer browsers support the ECMAScript Internationalization API, which has an optionally implemented function to return the time zone. It may work in some browsers, but is not guaranteed to return a valid result everywhere.
Intl.DateTimeFormat().resolvedOptions().timeZone
Again, you'll need Noda Time on the back end.
You said:
The problem is, if when I change timezone of client machine to (UTC -8:00), the client machine shows the timezone name as "(UTC-08:00) Pacific Time (US & Canada)" but in application the timezone differs from the client system it shows as "(UTC-08:00) Baja California".
This is probably related to how you are choosing a time zone in your application code. It sounds to me like you're scanning the list of server time zones and choosing the first one that matches some criteria. Since both of these time zones have the same base offset, you're probably just picking the wrong one, and you shouldn't be doing that anyway. But since you didn't show that part of your code, I can't help you much there.
To answer your specific questions:
Whether the TimeZoneInfo class can work automatically work according to the adjusment rule, when we convert from UTC to Local?
Yes, it can. There's nothing wrong with TimeZoneInfo, it's all about how you're using it. You probably are selecting the wrong time zone.
Do we have to detect the DST for particular datetime using the method TimeZoneInfoObject.IsDaylightSavingTime(DateTime) and do conversion?
No, you should not have to do that just to convert from UTC to a specific time zone. The ConvertTimeFromUtc function will handle that for you.
Is there any other classes in .Net which can sync with windows timezones?
TimeZoneInfo is the only one built in to the .NET Framework. Noda Time is a great alternative that can work with either Windows time zone or IANA time zones.
Lastly, I'll re-iterate what Jon said in comments. If all you're doing is displaying a particular instant in time to an end-user, then forget about time zone detection or working with local time on the server at all. Just send the UTC time to the client, and use either the UTC functions on the JavaScript Date object, or use a library like moment.js. Either can work in both UTC and local, and can convert between them. For example (using moment.js):
var valueFromServer = "2015-07-26T12:00:00Z"; // the Z means UTC
var localTime = moment(valueFromServer).format(); // "2015-07-26T05:00:00-07:00" (Pacific)
To keep it short and sweet I have 2 PC's:
PC 1 has DST switched off
PC 2 has DST switched on.
PC 1 sends a DateTime to PC 2 which it takes and uses to set its time.
All the above I have in place but my questions is that when PC 2 receives the DateTime how can I check to see if it needs DST adjustments?
PC's are UK based.
EDIT:- Bit more detail incase there is confusion.
When PC 2 retrieves the time from PC 1 it will change the system's time to this value but I need to ensure that if a +/- 1 hour is required (DTS) then it is applied before setting the system date and time.
I would recommend using UTC for transport and persistance.
Also take care to not use date time as a critical part of your algorithm - it's just data. Dates and time shift and correct themselves. I've seen apps that crash when the PC corrects it's time :)
Here's another post with more information on the topic:
Daylight saving time and time zone best practices
You didn't show any code, but you tagged your question as c#, so I will answer from that perspective.
If you just need to make sure you're talking about the same moment in time, then use a DateTime with .Kind = DateTimeKind.Utc, such as is obtained from DateTime.UtcNow.
If you actually need to know that PC1 thought it was one time while PC2 thought it was another, and you still want to know that those two things represent the same moment in time, then use a DateTimeOffset. This will include the local date and time of the computer, as well as the offset from UTC that the date and time represent.
With either approach, you should use the ISO8601 format for transport and persistence, so that your meaning is clear regardless of culture or time zone of the observer. In .Net, this is obtained with .ToString("o") from either a DateTime or DateTimeOffset.
A UTC DateTime in ISO8601 format would look like this:
2013-03-06T09:00:00Z
A DateTimeOffset in ISO8601 format would look like this:
2013-03-06T10:00:00+01:00
.Net calls this the Round Trip pattern - because it is designed for the exact purpose you described.
For further clarification on DateTimeOffset - see this post.