为什么UtcDateTime函数不将偏移量添加到UTC日期？

Question

For instantaneous DateTime tracking, I am using a DateTimeOffset datatype. The following function adds the user corresponding TimeZone ID offset to the UTC DateTime property of DateTimeOffset

According to the documentation , UtcDateTime will perform both a time zone conversion and a type conversion on a DateTimeOffset . The following code does not though. Why is the conversion not taking place?

Function to add TimeSpan offset,

public static DateTimeOffset GetUtcDateTime (DateTime sourceDateTime, string timeZoneId) {
 TimeZoneInfo timeZone = TimeZoneInfo.FindSystemTimeZoneById (timeZoneId);
 TimeSpan offset = timeZone.GetUtcOffset (sourceDateTime);
 DateTimeOffset utcTime = new DateTimeOffset (sourceDateTime, offset);
 return utcTime;
 }

and here where I am trying to convert,

DateTimeOffset utcDate = (DateTime.UtcNow);
DateTime fromUtc = utcDate.DateTime;
DateTimeOffset UtcDate = StaticHandlers.GetUtcDateTime (fromUtc, "America/Los_Angeles");
Console.WriteLine ("UTC now is {0} and UTC Date LA is {1} and UtcDateTime LA is {2}", utcDate, UtcDate, utcDate.UtcDateTime);

the output is,

UTC now is 5/8/18 6:43:37 AM +00:00 and and UTC Date LA is 5/8/18 6:43:37 AM -07:00 UtcDateTime LA is 5/8/18 6:43:37 AM

update,

I want to preserve both UTC and the user offset for tracking purposes. DST matters in this context. The example below shows what I am talking about.

DateTime currentDateTime = DateTime.Now;
DateTime beforeDST_LA = new DateTime (2018, 3, 11, 0, 0, 0);
DateTime afterDST_LA = new DateTime (2018, 3, 12, 0, 0, 0);
TimeSpan offsetCurrent = tzi.GetUtcOffset (currentDateTime);
TimeSpan offsetBeforeDST = tzi.GetUtcOffset (beforeDST_LA);
TimeSpan offsetAfterDST = tzi.GetUtcOffset (afterDST_LA);
Console.WriteLine ("Current offset is {0} before DST is {1} and After DST is {2}", offsetCurrent, offsetBeforeDST, offsetAfterDST);

Current offset is -07:00:00 before DST is -08:00:00 and After DST is -07:00:00

Answer 1

First, I would not call your function GetUtcDateTime , because that's not what it does. It is trying to get a DateTimeOffset for a specific time zone for a specific time, so call it something like GetDateTimeOffset .

The main concept you're missing in your code is that DateTime has .Kind property, which sets a DateTimeKind value. The kind is taken into consideration by several places in your code:

• GetUtcOffset will convert Utc or Local kinds to the zone provided before determining the offset.

• new DateTimeOffset (the constructor) will error if the kind and the offset conflict, if you provide an offset.

• When you assign a DateTime to a DateTimeOffset , the implicit conversion is evaluating the kind.

• When you call .DateTime from the DateTimeOffset , the kind will always be Unspecified - regardless of the offset.

If you take all of this into account, you'll realize you need to check the kind yourself before calling GetUtcOffset . If it's not Unspecified then you'll need to convert it to the specified time zone before getting the offset.

public static DateTimeOffset GetDateTimeOffset(DateTime sourceDateTime, string timeZoneId)
{
    TimeZoneInfo timeZone = TimeZoneInfo.FindSystemTimeZoneById(timeZoneId);

    // here, is where you need to convert
    if (sourceDateTime.Kind != DateTimeKind.Unspecified)
        sourceDateTime = TimeZoneInfo.ConvertTime(sourceDateTime, timeZone);

    TimeSpan offset = timeZone.GetUtcOffset(sourceDateTime);
    return new DateTimeOffset(sourceDateTime, offset);
}

Now that this is handled, turn to the next set of problems, which is where you call it.

DateTimeOffset utcDate = (DateTime.UtcNow);
DateTime fromUtc = utcDate.DateTime;

In line 1, the implicit cast from DateTime to DateTimeOffset sets the offset to 00:00 - because DateTime.UtcNow has .Kind == DateTimeKind.Utc .

In line 2, the call to the .DateTime property sets fromUtc.Kind == DateTimeKind.Unspecified . Essentially, you've stripped away the kind.

So instead of this, just pass DateTime.UtcNow directly into the function. The kind will persist, and it will all work - now that the Kind is recognized and the conversion is happening inside the function.

All that said, if your original values are all DateTimeOffset (example, DateTimeOffset.UtcNow ) then you don't need that function at all. Just call TimeZoneInfo.ConvertTime with the DateTimeOffset directly.