C# 将特定时区的 1 天添加到 DateTimeOffset

Question

我有一个 DateTimeOffset 实例，考虑到夏令时规则(所以它可能会导致 Offset 改变)。如果没有第 3 方库，我该怎么做？

可验证的例子:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace UnitTestProject
{
    [TestClass]
    public class TimeZoneTests
    {
        [TestMethod]
        public void DateTimeOffsetAddDays_DaylightSaving_OffsetChange()
        {
            var timeZoneId = "W. Europe Standard Time";
            var utcTimestamp = new DateTimeOffset(2017, 10, 28, 22, 0, 0, TimeZoneInfo.Utc.BaseUtcOffset);
            var weuropeStandardTimeTimestamp = TimeZoneInfo.ConvertTimeBySystemTimeZoneId(utcTimestamp, timeZoneId);
            Assert.AreEqual(new DateTime(2017, 10, 29), weuropeStandardTimeTimestamp.DateTime);
            Assert.AreEqual(TimeSpan.FromHours(2), weuropeStandardTimeTimestamp.Offset);

            var weuropeStandardTimeTimestampNextDay = AddDaysInTimeZone(weuropeStandardTimeTimestamp, 1, timeZoneId);

            Assert.AreEqual(new DateTime(2017, 10, 30), weuropeStandardTimeTimestampNextDay);
            Assert.AreEqual(TimeSpan.FromHours(1), weuropeStandardTimeTimestamp.Offset);
        }

        private DateTimeOffset AddDaysInTimeZone(DateTimeOffset timestamp, int days, string timeZoneId)
        {
            // this line has to be fixed:
            return timestamp.AddDays(days);
        }
    }
}

AddDaysInTimeZone 方法应替换为正确的实现。

PS 如果它导致无效/不明确/跳过的日期，那么可以抛出异常。

Answer 1

TimeZoneInfo 使这个合理简单 - 只需将一天添加到值的 DateTime 部分，检查结果是否被跳过或有歧义，如果不是，请向区域询问 UTC 偏移量。这是一个显示所有不同可能性的完整示例:

using System;
using System.Globalization;

using static System.FormattableString;

class Program
{
    static void Main()
    {        
        // Stay in winter
        Test("2017-01-22T15:00:00+01:00");
        // Skipped time during transition
        Test("2017-03-25T02:30:00+01:00");
        // Offset change to summer
        Test("2017-03-25T15:00:00+01:00");
        // Stay in summer
        Test("2017-06-22T15:00:00+02:00");
        // Ambiguous time during transition
        Test("2017-10-28T02:30:00+02:00");
        // Offset change back to winter
        Test("2017-10-28T15:00:00+02:00");
        // Stay in winter
        Test("2017-12-22T15:00:00+01:00");
    }

    static void Test(string startText)
    {
        var zone = TimeZoneInfo.FindSystemTimeZoneById("W. Europe Standard Time");
        var start = DateTimeOffset.ParseExact(
            startText, "yyyy-MM-dd'T'HH:mm:ssK", CultureInfo.InvariantCulture);
        try
        {
            var end = AddOneDay(start, zone);
            Console.WriteLine(Invariant($"{startText} => {end:yyyy-MM-dd'T'HH:mm:ssK}"));
        }
        catch (Exception e)
        {
            Console.WriteLine($"{startText} => {e.Message}");
        }
    }

    static DateTimeOffset AddOneDay(DateTimeOffset start, TimeZoneInfo zone)
    {
        var newLocal = start.DateTime.AddDays(1);
        // TODO: Use a better exception type :)
        if (zone.IsAmbiguousTime(newLocal))
        {
            throw new Exception("Ambiguous");
        }
        if (zone.IsInvalidTime(newLocal))
        {
            throw new Exception("Skipped");
        }
        return new DateTimeOffset(newLocal, zone.GetUtcOffset(newLocal));
    }

}