如何从 UTC 获取日期时间的偏移量？

Question

给定 “本地” 的日期时间（即没有时区信息），例如：

| Datetime            |
|---------------------|
| 2019-01-21 09:00:00 |
| 2019-02-21 09:00:00 |
| 2019-03-21 09:00:00 |
| 2019-04-21 09:00:00 |
| 2019-05-21 09:00:00 |
| 2019-06-21 09:00:00 |
| 2019-07-21 09:00:00 |
| 2019-08-21 09:00:00 |
| 2019-09-21 09:00:00 |
| 2019-10-21 09:00:00 |
| 2019-11-21 09:00:00 |
| 2019-12-21 09:00:00 |

我怎样才能得到该日期与 UTC 的偏移量？（假设机器本地时区信息）

例如，我的本地 PC 处于东部时区。东部时区是：

300 分钟（5 小时）落后于 UTC
240 分钟（4 小时）落后于 UTC

取决于“夏令时”是否在该日期时间生效。

这意味着上面的列表：

| Datetime            | Offset from UTC (minutes)  |
|---------------------|----------------------------|
| 2019-01-21 09:00:00 | -300  (-5 hours)           |
| 2019-02-21 09:00:00 | -300  (-5 hours)           |
| 2019-03-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-04-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-05-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-06-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-07-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-08-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-09-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-10-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2019-11-21 09:00:00 | -300  (-5 hours)           | 
| 2019-12-21 09:00:00 | -300  (-5 hours)           |

当然，如果日期 are from before 2007，这些偏移量也会改变，答案也会改变：

| Datetime            | Offset from UTC (minutes)  |
|---------------------|----------------------------|
| 2006-01-21 09:00:00 | -300  (-5 hours)           |
| 2006-02-21 09:00:00 | -300  (-5 hours)           |
| 2006-03-21 09:00:00 | -240  (-5 hours)           |
| 2006-04-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-05-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-06-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-07-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-08-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-09-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 2006-10-21 09:00:00 | -240  (-5 hours)           |
| 2006-11-21 09:00:00 | -300  (-5 hours)           | 
| 2006-12-21 09:00:00 | -300  (-5 hours)           |

在 1977 年的能源危机期间，答案又会有所不同，因为国家运行全年实行夏令时：

| Datetime            | Offset from UTC (minutes)  |
|---------------------|----------------------------|
| 1977-01-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-02-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-03-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-04-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-05-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-06-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-07-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-08-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-09-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-10-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-11-21 09:00:00 | -240  (-4 hours)           | Daylight savings
| 1977-12-21 09:00:00 | -240  (-4 hours)           | Daylight savings

并且 before 1966 答案有一些变化。

Windows 知道所有这些事情。

所以问题是：

给定 FILETIME 格式的日期时间
假定为当前PC的时区
我怎样才能得到日期时间与 UTC 的偏移量
在日期时间

换句话说：

//Pesudocode. It may look like C#, but i'm using the native Win32 api
Int32 GetDateTimeMinutesOffsetFromUTC(DateTime value)
{
   //2006-03-21 09:00:00 ==> -300
   //2007-03-21 09:00:00 ==> -240
   return -1; //todo
}

或

function GetDateTimeMinutesOffsetFromUtc(Value: TDateTime): Integer;
begin
   //2006-03-21 09:00:00 ==> -300
   //2007-03-21 09:00:00 ==> -240
   Result := -1; //todo
end;

或

int GetDateTimeMinutesOffsetFromUtc(FILETIME value)
{
   //2006-03-21 09:00:00 ==> -300
   //2007-03-21 09:00:00 ==> -240
   Result := -1; //todo
}

提醒一下，我使用的是 Win32 api。

这不是 C/C++（即我无权访问 C 标准库）
这不是 C#（即我无权访问 .NET Framework Class 库）
这不是 Java（即我无权访问 Java Class 图书馆）
这不是Python
这不是 Javascript、React、Rust、Django

我说的是 Windows 和 Win32 API.

SQL 服务器

以上工作可以在SQL服务器中看到：

SELECT
    EventDate, 
    DATEDIFF(minute, CAST(EventDate AS datetime) AT TIME ZONE 'Eastern Standard Time', EventDate) AS MinutesOffsetFromUTC
FROM (VALUES
    ('2019-01-21 09:00:00.000'),
    ('2019-02-21 09:00:00.000'), 
    ('2019-03-21 09:00:00.000'), 
    ('2019-04-21 09:00:00.000'), 
    ('2019-05-21 09:00:00.000'), 
    ('2019-06-21 09:00:00.000'), 
    ('2019-07-21 09:00:00.000'), 
    ('2019-08-21 09:00:00.000'), 
    ('2019-09-21 09:00:00.000'), 
    ('2019-10-21 09:00:00.000'), 
    ('2019-11-21 09:00:00.000'), 
    ('2019-12-21 09:00:00.000')
) foo(EventDate)

EventDate               MinutesOffsetFromUTC
----------------------- --------------------
2019-01-21 09:00:00.000 -300
2019-02-21 09:00:00.000 -300
2019-03-21 09:00:00.000 -240
2019-04-21 09:00:00.000 -240
2019-05-21 09:00:00.000 -240
2019-06-21 09:00:00.000 -240
2019-07-21 09:00:00.000 -240
2019-08-21 09:00:00.000 -240
2019-09-21 09:00:00.000 -240
2019-10-21 09:00:00.000 -240
2019-11-21 09:00:00.000 -300
2019-12-21 09:00:00.000 -300

(12 rows affected)

研究工作

大多数 Winapi 函数用于将 "local" 转换为 "UTC"，然后再返回，不要'考虑相关日期；但只使用 whether daylight savings is in effect right now:

Functions like FileTimeToLocalFileTime apply the current Daylight Savings Time (DST) bias rather than the bias that was in effect at the time in question.

其他人考虑转换的日期，但只看现在的夏令时开始和结束规则 - 而不是当时的规则。

但是 TzSpecificLocalTimeToSystemTime 是理解日期时间和夏令时的一个函数：

TzSpecificLocalTimeToSystemTime takes into account whether daylight saving time (DST) is in effect for the local time to be converted.

实际上 Windows 并不知道一切。它在注册表中保留历史 “夏令时” 日期的数据库：

所以对我来说它真正知道的是

2007年巨变前
及之后

但这对我的用例来说已经足够好了。它是 good enough for SQL Server.

红利阅读

Raymond Chens blog posts tagged with time

Answer 1

好的，时效已过。我给了每个人自己回答问题的机会，提供了各种提示，这样别人就可以得到甜蜜的声誉。

那个时代已经过去了。现在是回答问题的时候了because it's what Joel and Jeff would have wanted.

伪代码：

int GetDateTimeOffsetFromUtcMinutes(DateTime localDateTime)
{
   //All code on Whosebug is public domain; no attribution is ever required.

   /*
    Given a datetime, tell me how many minutes offset it was from UTC.

        2006-03-21 09:00:00 ==> -300  (before daylight savings rules changed)
        2007-03-21 09:00:00 ==> -240

    The problem is that we don't want to use the current setting of Daylight Savings or not.
    We want use if daylight savings was in effect *at the date* being supplied.

    And we can't even use the current rules:

            - Second Sunday in March: spring forward  (e.g. 3/14/2021 2:00 AM)
            - First Sunday in November: fall back     (e.g. 11/7/2021 2:00 AM)

    because those are the rules today.

    We need to use the rules that were in effect of the date we are considering.

    - e.g. the rules changed in 2007. If we have an OrderDate from 2006, we need those older rules.

    Also notice that some dates have two answers:

        11/7/2021 1:45 AM: EDT (-4 hours)
        11/7/2021 1:45 AM: EST (-5 hours, because at 2am we fallback to 1am, and encounter 1:45AM again, but this time as Standard time)

    So which one do we return? Whatever one i feel like. That's the price you pay for not using UTC or datetime's with an offset.
    */

   //Convert the date to a SYSTEM_TIME structure so we can call the Win32 API 
   SYSTEM_TIME stLocal;
   DateTimeToSystemTime(LocalDateTime, out stLocal);

   SYSTEM_TIME stUtc;
    
   if (!TzSpecificLocalTimeToSystemTime(null, stLocal, out stUtc))
       RaiseLastWin32Error();

   //We now have both "local" and "utc" as a SYSTEM_TIME.
   //Convert both to FILETIME so we can subtract them.
   FILETIME ftLocal, ftUtc;
   if (!SystemTimeToFileTime(stLocal, out ftLocal))
      RaiseLastWin32Error();
   if (!SystemTimeToFileTime(stUtc, out ftUtc))
      RaiseLastWin32Error();

   //Convert the FILETIMEs into Int64s. 
   //We do this because, as you know, you cannot access FILE_TIME structure
   //as an 64-bit integer, even though it is two 32-bit integers back to back.
   LARGE_INTEGER ulLocal, ulUtc;
   ulLocal.LowPart  = ftLocal.dwLowDateTime;
   ulLocal.HighPart = ftLocal.dwHighDatetime;

   ulUtc.LowPart  = ftUtc.dwLowDateTime;
   ulUtc.HighPart = ftUtc.dwHighDatetime;

   //Now subtract the quadparts
   Int64 delta = ulLocal.QuadPart - ulUtc.QuadPart;

   //That delta is in 100ns intervals (0.00001 sec). We want it in whole minutes;
   //         100 ns
   //       0.1 us
   //    0.0001 ms
   // 0.0000001 s
   delta = delta div 10000000; //100 ns ==> seconds  (div is integer division)
   delta = delta div 60; //seconds ==> minutes

   return delta;
}

然后，当然，没有测试用例，任何功能都是不完整的

//After the Daylight Savings rule change of of 2007
Test("2019-01-21T09:00:00", -300); //  (-5 hours)           |
Test("2019-02-21T09:00:00", -300); //  (-5 hours)           |
Test("2019-03-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-04-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-05-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-06-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-07-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-08-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-09-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-10-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2019-11-21T09:00:00", -300); //  (-5 hours)           |
Test("2019-12-21T09:00:00", -300); //  (-5 hours)           |

//Before the Daylight Savings rule change of 2007
Test("2006-01-21T09:00:00", -300); //  (-5 hours)           |
Test("2006-02-21T09:00:00", -300); //  (-5 hours)           |
Test("2006-03-21T09:00:00", -300); //  (-5 hours)           | What what?
Test("2006-04-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-05-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-06-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-07-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-08-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-09-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-10-21T09:00:00", -240); //  (-4 hours)           | Daylight savings
Test("2006-11-21T09:00:00", -300); //  (-5 hours)           |
Test("2006-12-21T09:00:00", -300); //  (-5 hours)           |

//Testing spring-forward. Spring forward March 14, 2021 at 2:00 AM
//The weirdness here is that the time from 2:00:00..2:59:59 doesn't exist.
Test("2021-03-14T00:00:00", -300); // EST
Test("2021-03-14T00:59:59", -300); // EST
Test("2021-03-14T01:00:00", -300); // EST
Test("2021-03-14T01:59:00", -300); // EST
Test("2021-03-14T02:00:00", -240); // There is no March 14, 2021 2am - it doesn't exist. The clock goes from 1:59:59 am --> 3:00:00 am. The function does return 240. TODO: figure out why it returns 240
Test("2021-03-14T02:59:59", -240); // There is no March 14, 2021 2:59am - it doesn't exist.
Test("2021-03-14T03:00:00", -240); // EDT

//Testing fall-back. Fall back March 14, 2021 at 2:00 AM
//The weirdness here is that 1:30 AM exists twice.
//12:00 AM -> 12:59:59 AM -> [1:00 AM -> 1:59:59 AM --> 1:00 AM -> 1:59:59 AM] -> 2:00 AM
//So there's no way to know if 11/7/2021 1:30 AM was EDT or EST - both are correct, because it actually did happen twice.
Test("2021-11-07T00:00:00", -240); // EDT
Test("2021-11-07T00:59:59", -240); // EDT
Test("2021-11-07T01:00:00", -240); // EDT (and EST!)
Test("2021-11-07T01:59:59", -240); // EDT (and EST!)
//Test("2021-11-07T01:00:00", -300); // EST (and EDT!)
//Test("2021-11-07T01:59:59", -300); // EST (and EDT!)
Test("2021-11-07T02:00:00", -300); // EST

如何从 UTC 获取日期时间的偏移量？

How to get a datetime's offset from UTC?

winapi

timezone

SQL 服务器

研究工作

红利阅读