Question

我使用以下代码显示2005年到2035年之间的夏令时。

对于2005年，这个page显示DST是在4月3日到10月30日之间。但GetDaylightChanges将于3月13日和11月6日返回。

.NET GetDaylightChanges是一个可靠的函数吗？

enter image description here

    public static void GetCurrentTimeZone()
    {
        for (int i = 0; i < 30; i++)
        {
            var dlt = TimeZone.CurrentTimeZone.GetDaylightChanges(2005 + i);

            Console.WriteLine(2005 + i);
            Console.WriteLine(dlt.Start.ToLongDateString());
            Console.WriteLine(dlt.End.ToLongDateString());

            Console.WriteLine(" ");
        }
    }

Answer 1

从1987年到2006年，规则是：DST从4月的第一个星期日到10月的最后一个星期日生效。

从2007年至今，规则是：DST从3月的第二个星期日到11月的第一个星期日生效。

然而，正如@MaheshKava所指出的， GetDaylightChanges API的备注部分说明了这一点：

因为TimeZone类仅支持一个夏令时调整规则，GetDaylightChanges方法应用当前调整规则到任何一年，无论是否调整规则实际上适用于那一年。

这意味着当前规则（3月的第2个星期日至11月的第1个星期日）适用于所有年份，无论此规则是否在该年度实际生效。总而言之， GetDaylightChanges 会在2007年之前的任何一年给你不准确的结果，显然这是设计的。

API文档继续说明您可以使用 TimeZoneInfo.GetAdjustmentRules 来获取更准确的信息。我很好奇，所以我写了这段代码：

    static void Main(string[] args)
    {
        PrintAllDaylightSavingsAdjustmentDates();
        Console.ReadLine();
    }

    public static void PrintAllDaylightSavingsAdjustmentDates()
    {
        TimeZoneInfo timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
        TimeZoneInfo.AdjustmentRule[] adjustmentRules = timeZoneInfo.GetAdjustmentRules();

        for (int year = 2000; year < 2030; year++)
        {
            PrintDaylightSavingsAdjustmentDatesForYear(adjustmentRules, year);
        }
    }

    public static void PrintDaylightSavingsAdjustmentDatesForYear
        (
        TimeZoneInfo.AdjustmentRule[] adjustmentRules, 
        int year
        )
    {
        DateTime firstOfYear = new DateTime(year, 1, 1);

        foreach (TimeZoneInfo.AdjustmentRule adjustmentRule in adjustmentRules)
        {
            if ((adjustmentRule.DateStart <= firstOfYear) && (firstOfYear <= adjustmentRule.DateEnd))
            {
                Console.WriteLine("In {0}, DST started on {1} and ended on {2}.",
                    year,
                    GetTransitionDate(adjustmentRule.DaylightTransitionStart, year).ToString("MMMM dd"),
                    GetTransitionDate(adjustmentRule.DaylightTransitionEnd, year).ToString("MMMM dd"));
            }
        }
    }

    public static DateTime GetTransitionDate
        (
        TimeZoneInfo.TransitionTime transitionTime,
        int year
        )
    {
        if (transitionTime.IsFixedDateRule)
        {
            return new DateTime(year, transitionTime.Month, transitionTime.Day);
        }
        else
        {
            if (transitionTime.Week == 5)
            {
                // Special value meaning the last DayOfWeek (e.g., Sunday) in the month.
                DateTime transitionDate = new DateTime(year, transitionTime.Month, 1);
                transitionDate = transitionDate.AddMonths(1);

                transitionDate = transitionDate.AddDays(-1);
                while (transitionDate.DayOfWeek != transitionTime.DayOfWeek)
                {
                    transitionDate = transitionDate.AddDays(-1);
                }

                return transitionDate;
            }
            else
            {
                DateTime transitionDate = new DateTime(year, transitionTime.Month, 1);
                transitionDate = transitionDate.AddDays(-1);

                for (int howManyWeeks = 0; howManyWeeks < transitionTime.Week; howManyWeeks++)
                {
                    transitionDate = transitionDate.AddDays(1);
                    while (transitionDate.DayOfWeek != transitionTime.DayOfWeek)
                    {
                        transitionDate = transitionDate.AddDays(1);
                    }
                }

                return transitionDate;
            }
        }
    }

其中吐出以下输出：

In 2000, DST started on April 02 and ended on October 29.
In 2001, DST started on April 01 and ended on October 28.
In 2002, DST started on April 07 and ended on October 27.
In 2003, DST started on April 06 and ended on October 26.
In 2004, DST started on April 04 and ended on October 31.
In 2005, DST started on April 03 and ended on October 30.
In 2006, DST started on April 02 and ended on October 29.
In 2007, DST started on March 11 and ended on November 04.
In 2008, DST started on March 09 and ended on November 02.
In 2009, DST started on March 08 and ended on November 01.
In 2010, DST started on March 14 and ended on November 07.
In 2011, DST started on March 13 and ended on November 06.
In 2012, DST started on March 11 and ended on November 04.
In 2013, DST started on March 10 and ended on November 03.
In 2014, DST started on March 09 and ended on November 02.
In 2015, DST started on March 08 and ended on November 01.
In 2016, DST started on March 13 and ended on November 06.
In 2017, DST started on March 12 and ended on November 05.
In 2018, DST started on March 11 and ended on November 04.
In 2019, DST started on March 10 and ended on November 03.
In 2020, DST started on March 08 and ended on November 01.
In 2021, DST started on March 14 and ended on November 07.
In 2022, DST started on March 13 and ended on November 06.
In 2023, DST started on March 12 and ended on November 05.
In 2024, DST started on March 10 and ended on November 03.
In 2025, DST started on March 09 and ended on November 02.
In 2026, DST started on March 08 and ended on November 01.
In 2027, DST started on March 14 and ended on November 07.
In 2028, DST started on March 12 and ended on November 05.
In 2029, DST started on March 11 and ended on November 04.

.NET TimeZone.CurrentTimeZone.GetDaylightChanges返回错误的2005年DST

1 个答案: