添加 TimeSpan 的 DateTimeOffset 返回其 TimeZoneInfo 的无效 UTC 偏移量

Question

我正在构建一个时间表达式库，该库必须正确全球化，因此可以在所有可用时区工作。

现在我似乎陷入了困境，因为我不确定当使用任何种类的 .Add 移动天数、小时数等跨越过渡边界时，如何在正确的夏令时 (DST) 中从 DateTimeOffset 对象检索调整后的日期。

有趣的是，我找到了本地系统时区的解决方法，但还没有找到任何方法将相同的策略应用于任何任意时区。

我能够找到一个片段（没有保留源代码，抱歉！），它尝试按偏移量反向查找时区信息，但由于存在多个潜在结果，每个结果可能都有不同的 DST 规则，这些规则不起作用。 （可能有一些可用的优化，但我认为基本前提有缺陷）

public TimeZoneInfo GetTimeZoneInfo(DateTimeOffset Value)
{
    // Search available sytem time zones for a matching one
    foreach (var tzi in TimeZoneInfo.GetSystemTimeZones())
    {
        // Compare value offset with time zone offset
        if (tzi.GetUtcOffset(Value).Equals(Value.Offset))
        {
            return tzi;
        }
    }
}

证明这个东西可能有点乏味，所以我将核心问题提取到几个方法和单元测试中，希望能够证明我正在解决的问题面对。

public DateTimeOffset GetNextDay_Wrong(DateTimeOffset FromDateTimeOffset)
{
    // Cannot create a new DateTimeOffset using simply the supplied value's UtcOffset
    // because in PST, for example, it could be -7 or -8 depending on DST
    return new DateTimeOffset(FromDateTimeOffset.Date.AddDays(1), FromDateTimeOffset.Offset);
}

[TestMethod]
public void GetNextDay_WrongTest()
{
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_Wrong(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_Wrong(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

public DateTimeOffset GetNextDay_LooksRight(DateTimeOffset FromDateTimeOffset)
{
    // Because we cannot create a new DateTimeOffset we simply adjust the one provided!
    var temp = FromDateTimeOffset;
    // Move back to midnight of the current day
    temp = temp.Subtract(new TimeSpan(temp.Hour, temp.Minute, temp.Second));
    // Now move to the next day
    temp = temp.AddDays(1);
    // Let the DateTimeOffset class do it's magic
    temp = temp.ToLocalTime();
    // Check if the time zone has changed
    if (FromDateTimeOffset.Offset != temp.Offset)
    {
        // Calculate the change amount (could be 30 mins or even stranger)
        var delta = FromDateTimeOffset.Offset - temp.Offset;
        // Adjust the temp value by the delta
        temp = temp.Add(delta);
    }
    return temp.ToLocalTime();
}

[TestMethod]
public void GetNextDay_LooksRightTest()
{
    // Everything is looking good and the test passes now, so we're home free yeah?

    // { To work this needs to match your system's configured Local Time Zone, I'm in PST }
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_LooksRight(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_LooksRight(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

[TestMethod]
public void GetNextDay_LooksRight_FAILTest()
{
    // Here is where the frustrating part is... aparantly the "magic" that DateTimeOffset provides only works for your systems Local Time Zone...

    // { To properly fail this cannot match your system's configured Local Time Zone, I'm in PST so I use EST }
    var tz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

    var workingDate = new DateTime(2009, 11, 2, 0, 0, 0);
    var failingDate = new DateTime(2009, 11, 1, 0, 0, 0);

    var workingDate_tz = new DateTimeOffset(workingDate, tz.GetUtcOffset(workingDate));
    var failingDate_tz = new DateTimeOffset(failingDate, tz.GetUtcOffset(failingDate));

    var actual_workingDate_tz = GetNextDay_LooksRight(workingDate_tz);
    var actual_failingDate_tz = GetNextDay_LooksRight(failingDate_tz);

    var expected_workingDate = new DateTime(2009, 11, 3, 0, 0, 0);
    var expected_failingDate = new DateTime(2009, 11, 2, 0, 0, 0);

    var expected_workingDate_tz = new DateTimeOffset(expected_workingDate, tz.GetUtcOffset(expected_workingDate));
    var expected_failingDate_tz = new DateTimeOffset(expected_failingDate, tz.GetUtcOffset(expected_failingDate));

    Assert.AreEqual(expected_workingDate_tz, actual_workingDate_tz, "Should have found the following day's midnight");
    Assert.AreEqual(expected_failingDate_tz, actual_failingDate_tz, "Failing date does not have the correct offset for it's DST");
}

Answer 1

此案例的根本问题是没有足够的信息来执行适当的时区转换。简单的偏移量不足以具体地推断时区，因为对于特定时刻的给定偏移量，可能存在多个潜在的时区。因为 DateTimeOffset 对象不捕获和维护有关它创建时的时区的信息，所以必须由该类外部的某些东西负责维护这种关系。

让我感到困惑的是调用 ToLocalTime() ，我后来意识到它实际上是在计算中引入了隐含的 TimeZoneInfo ，即机器配置的本地时间，并且我相信在内部 DateTimeOffset 必须转换为 UTC只需删除配置的偏移量，然后使用构造函数创建一个新的 DateTimeOffset 类，该构造函数采用 DateTime [UTC 格式] 和 TimeZoneInfo [来自本地系统] 来生成正确的夏令时感知结果日期。

鉴于此限制，我不再在 DateTimeOffset 类中看到任何比 DateTime 和 TimeZoneInfo 的同样准确且更有价值的组合更有价值的值。