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I have a date, say "2016-10-20 20:34:60". When I create this datetime, I'm not sure how to specify that this DateTime was made based on the servers local timezone. IN this case, it's "Pacific Standard Time".

I want to present this as both a default text and data text. This data text will be used via javascript to change the date into the user's local time.

I'm new to C#, and am trying to wrap my head around how to do so. Many of the examples given are using UTC time. We store dates based on the servers localtime.

I want to ensure that the displayed default text is correct, regardless if we are in daylight saving mode or not, it should reflect what it was at that moment in time.

Sam
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  • This should help: https://stackoverflow.com/questions/3323113/datetime-to-string-with-time-zone – Cory Oct 10 '18 at 18:36
  • As an aside, you may wish to consider using UTC. https://stackoverflow.com/questions/11537106/is-it-always-a-good-idea-to-store-time-in-utc-or-is-this-the-case-where-storing – MikeH Oct 10 '18 at 18:41
  • @MikeH, I wish they stored things as UTC. makes everything easier. But I have no control of this. – Sam Oct 10 '18 at 18:54
  • @Sam - Just because non-UTC time is stored, doesn't mean you can't convert to UTC before sending it out the door. – Matt Johnson-Pint Oct 10 '18 at 21:20
  • @MattJohnson, I am sending the time as a UTC timestamp as a hidden field, which is what I use with javascript and the Date object. I was just concerned with the date returned from the DB and making sure that THAT was set to the proper timezone since I saw that Kind was Unspecified. – Sam Oct 10 '18 at 22:28
  • You can use `DateTime.SpecifyKind` to change the kind without changing the value. Though your only options are `Local`, `Utc`, and `Unspecified`. It's not a great API. Also keep in mind that math operations (`AddMinutes`, etc.) do not take DST transitions into account. – Matt Johnson-Pint Oct 11 '18 at 01:01

3 Answers3

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You can use 

    TimeZoneInfo timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("Pacific SA Standard Time");

DateTime newDateTime = TimeZoneInfo.ConvertTime(existingDateTime, timeZoneInfo);

You can find more information and examples here and all time zones here

rippergr
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  • This wouldn't be very 'scaleable' as you'd always need to define the timezone. – Cory Oct 10 '18 at 18:37
  • does ConvertTime simplay just change the DateTimeKind of the 'existingDateTime', keeping the same date? Thats basically what I'm trying to get. So that it shows the proper offset for that date at that given point in time. – Sam Oct 10 '18 at 18:56
  • FYI - `"Pacific SA Standard Time"` is for Santiago, Chile. Also, be careful about hard-coded lists of time zones on web pages. They change as the world does. Instead, use `TimeZoneInfo.GetSystemTimeZones()`, or use `TZUtil.exe /L` on the command line. Thanks. – Matt Johnson-Pint Oct 10 '18 at 20:52
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In general, you shouldn't rely on the server's local time zone. It's a system-wide setting that can be easily changed without administrative permissions. Doing so makes you vulnerable to hard to track down production bugs, such as one server in a cluster having a different time zone setting than another. It also presents a problem if ever moving the server to a different location (or to the cloud) and having your data affected.

It's also very common for servers to have their time zones set to UTC intentionally, as to not have their log files affected by things like daylight saving time transitions.

If regardless you still want to assert that a DateTime value reflected the computer's local time zone, you'd do that by ensuring DateTimeKind.Local is set in the .Kind property. This can be done in a constructor parameter, or with DateTimeStyles.AssumeLocal when parsing, or with DateTime.SpecifyKind on an existing value. You could then use .ToUniversalTime() to convert to UTC before sending it along to the end user.

However, I strongly encourage you to use DateTimeOffset instead. Unlike DateTime, it will keep the offset around, and take it into consideration for most operations. If you construct a DateTimeOffset from a DateTime with DateTimeKind.Unspecified, it will default to the local time zone. Similarly, if you create one via parsing (DateTimeOffset.Parse, .TryParse, .ParseExact, .TryParseExact, etc.) if there is no offset in the input string, it will default to the local time zone also.

HOWEVER - none of these objects will actually track a time zone in a way that will properly take transitions into account (such as when daylight saving time moves Pacific Time between PST and PDT and vice versa). If that's what you're looking for - sorry, you're out of luck with the built-in types. Instead you'll need to use the ZonedDateTime class from the Noda Time open-source library.

Also, you stated:

... This data text will be used via javascript to change the date into the user's local time.

It is not impossible, but it is quite difficult to get JavaScript to convert a date and time to the user's local time zone unless you also have the offset. Serializing a DateTimeOffset value back to browser will give you that for display, but if you're going to ask the user to change the value then you'll have to convert it back somehow.

It's all heck of a lot easier if you simply pass UTC values back and forth between the browser and the server. Then you can just convert UTC to Local time and vice versa in the browser. (At least for date+time values. Don't try to convert date-only values.)

Matt Johnson-Pint
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  • yes. sorry. my approach was to present a default value given the servers localtime (the "Displayed" text) Basically, I create this via C#. Along with this, I also convert the datetime into a utc date (Y-M-DTH:M:S offset) convert the timestamp in c# to a utc timestamp which is what i then set as a hidden input field (the "Data" value). I then take that "Data" value and create a new Date js object and then use that to create the appropriate "Displayed" text (assuming it parsed.). I should update my question to make it more clear as to what my problem was. – Sam Oct 10 '18 at 22:21
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You can get the current time zone with 

TimeZoneInfo.Local

Alternatively, you can get the current offset like this:

DateTimeOffset.Now

You'd need to store the relevant info in the database along side your date and time.

EDIT Per the comment below and https://learn.microsoft.com/en-us/dotnet/standard/datetime/ TimeZone.CurrentTimeZone has been superseded. Use TimeZoneInfo.Local instead. The answer was modified to reflect this.

MikeH
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