C# 将字符串解析为具有特定时区的日期时间

Question

I've been struggling with this for a bit so I'll punt to SO.

I have an app where I need to parse strings into datetimes. The strings look like this:

"201503131557"

(they will always be this format "yyyyMMddHHmm") and always be in Central Standard Time (or Central Daylight Time depending on the date)even though they don't specify it.

When I parse them I understand that it parses them into local time. I'm running on Azure PaaS and don't want to change it to run in CST just to support this operation.

How do i write code that works both locally and in Azure PaaS that will correctly parse these dates into DateTimes setting their timezone to CST?

Here is a quick unit test I wrote to prove Matt Johnson's answer.

    [TestMethod]
    public void DateTests()
    {
        var unSpecDt = DateTime.ParseExact("201503131557", "yyyyMMddHHmm", CultureInfo.InvariantCulture);
        var tz = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");


        var utcDt = TimeZoneInfo.ConvertTimeToUtc(unSpecDt, tz);

        var offset = tz.GetUtcOffset(unSpecDt);
        var dto =new DateTimeOffset(unSpecDt, offset);


        var cstDt = dto.DateTime;

        Assert.IsTrue(cstDt.Hour - utcDt.Hour == offset.Hours);
    }

Answer 1

To parse the string, since you have only the date and time, and you know the specific format the strings will be in, do this:

DateTime dt = DateTime.ParseExact("201503131557", "yyyyMMddHHmm", CultureInfo.InvariantCulture);

The resulting value will have its Kind property set to DateTimeKind.Unspecified (not local time, as you thought). That is to be expected, as you provided no information regarding how this timestamp is related to UTC or local time.

You said the value represents time in Central Standard Time. You'll need a TimeZoneInfo object that understands that time zone.

TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time");

Note that this identifier represents the Central Time observed in the USA, including both CST or CDT depending on which is in effect for the given value (despite the word "Standard" in its name). Also note that it valid on Windows operating systems only. If you wanted to use .NET Core on some other OS, then you'd need to pass the IANA identifier "America/Chicago" instead (or use my TimeZoneConverter library to use either identifier on any platform).

The next step is to figure out what you want to do with this value. You might do a few different things with it:

• If you want to convert it to the equivalent UTC value, represented as a DateTime , then you can do this:

   DateTime utc = TimeZoneInfo.ConvertTimeToUtc(dt, tz);

• If you want a DateTimeOffset representation which holds the input you gave and the offset from UTC as it related to US Central Time, then you can do this:

   TimeSpan offset = tz.GetUtcOffset(dt); DateTimeOffset dto = new DateTimeOffset(dt, offset);

  Keep in mind that it's possible for the input time is invalid or ambiguous if it falls near a DST transition. The GetUtcOffset method will return the standard offset in such cases. If you want different behavior, you have more code to write (out of scope for this post).

• There are other things you might do, all of which are provided by theTimeZoneInfo class.

Note that "Azure PaaS" might refer to a few different things, and while there is a setting called WEBSITE_TIME_ZONE in Azure App Service - I don't recommend you lean on it. Consider it a last resort to be used only when you can't control the code. In most cases, it is better off writing your code to never depend on the time zone setting of the system it runs on. That means never calling DateTime.Now , or TimeZoneInfo.Local , DateTime.ToLocalTime , or even DateTime.ToUniversalTime (since it converts from the local time zone), etc. Instead, rely upon methods that work explicitly with either UTC or a specific time zone or offset. Then you will never need to care about where your app is hosted.

Lastly, understand that neither the DateTime or DateTimeOffset types have any capability of understanding that a value is tied to a specific time zone. For that, you'd need to either write your own class, or look to the Noda Time library whose ZonedDateTime class provides such functionality.

Answer 2

You can use:

DateTime.ParseExact(...)

public static DateTime ParseExact (string s, string format, IFormatProvider provider);

This datetime do not have any metadata about timezone. You can convert to UTC and then you will be sure that you are able to convert to all timezones.

Answer 3

Use the DateTime.TryParseExact(...) method with DateTimeStyles.RoundtripKind to avoid conversion.

DateTime dt;
DateTime.TryParseExact("201503131557", "yyyyMMddHHmm", null, System.Globalization.DateTimeStyles.RoundtripKind, out dt);

If you need to convert you will need to go to UTC then use the conversion methods of TimeZoneInfo with TimeZoneInfo.FindSystemTimeZoneById("Central Standard Time") .