我不得不在我的应用程序中处理一些时间转换。我想尽可能坚持使用标准库函数。现在我使用time_t结构作为我的系统时基。但是，某些设备可以将时间同步到我的设备，该时间可能是也可能不是UTC。此外，我的设备将时间同步到另一台设备，该时间将始终为UTC。
无论如何，我可以询问用户什么时区是同步到我的设备的时间以及他们是否使用DST。当我的设备获得时间同步时，我可以使用mktime直接生成我的时间戳（我的设备系统时间需要与他们为时间戳目的同步的时间相匹配，否则我必须不断进行转换），然后我可以如果我知道我的时间同步来自非UTC源，则使用gmtime（）来获取UTC时间。问题是默认情况下localtime（）和gmtime（）将返回相同的值，因为默认情况下库认为它是在UTC时间内，并且没有DST或时区偏移。
所以，我认为解决这个问题的方法是实现并覆盖库__getzone函数。
来自EW430_CompilerReference.pdf的第106页
要使__time32，__ time64和date函数有效，必须实现 函数clock，__ time32，__ time64和__getzone。是否使用__time32 或__time64取决于您用于time_t的接口，请参见time.h，第304页。
...
__getzone的默认实现指定UTC（Coordinated Universal） 时间）作为时区。
问题1：在我的推理中，我是否走上了正确的道路，做我想做的最好的方法是实现这个__getzone函数？
我讨厌的原因是因为__getzone返回的值是一个奇怪的字符串w /格式如下：
：[XXX [：YYY [：NNN [：DST [：DST ...]]]]]
如果XXX是标准时区名称（例如EST的GMT-5），YYY是夏令时时区名称（例如EST的GMT-4），NNN是UTC的形式HHMM的数字偏移（可能有a - symbol）。，然后DST为夏令时规则指定一系列选项，这些规则有自己的恼人格式。
无论如何，这对我来说应该是非常直接的，因为我只担心加拿大和美国有相同的DST规则。
Q2：有没有人有任何形成该字符串的示例代码，以便检查我对此的理解？
答案 0 :(得分：2)
这是我对__getzone（）的实现。所以现在我的系统时基将是UTC。当用户配置我的系统时，如果时间源不提供UTC，我会询问他们当地的时间。然后，当他们向我的系统提供时间同步时，他们提供的时间将通过调用MKTIME（将考虑DST规则）转换为UTC。然后，当时间呈现给用户时，将通过调用localtime（）来完成。
我们在实现这个过程中学到的另一件事是IAR的MKTIME（）实现将调用__getzone（），但除非你将tm_isdst设置为'-1'，否则不会考虑DST规则。 -1调用MKTIME（）根据规则确定是否应用DST。
/*!
* \brief Overrides default library function __getzone to support different time
* zones and DST rules.
* \returns Pointer to a const string containing the timezone + dst rules
*
* This function supports all time zones and DST rules for the U.S. and Canada.
*
* \par IAR Notes
* The return value should be a string on the following form:
* \code
* :[XXX[:YYY[:NNN[:DST[:DST ...]]]]]
* \endcode
* \par
* Where \b XXX is the standard time-zone name, \b YYY is the daylight
* savings time-zone name, \b NNN is the time zone offset, and the DSTs
* are the daylight savings time rules. Daylight savings time will add
* one hour to the normal time.
* \par
* The time zone offset \b NNN is specified as a number relative to UTC,
* possibly negative (east is positive), on the format HHMM, where HH
* is hours and MM is minutes.
* \par
* The DSTs specifes a set of rules for how daylight savings time is
* applied. The rules must be sorted in increasing date order starting
* from the earliest date. The first rule for a specific year will
* enable DST, the next will disable it, and so on. Each rule is on
* the following form:
* \code
* [(YYYY)]MMDD[HH][-W|+W]
* \endcode
*
* * \b (YYYY) is the first year the daylight savings rule was applied.
* It is optional. If not specified it will default to the same
* year as the previous rule or zero if no previous rule.
* * \b MM is the month number (1-12).
* * \b DD is the day of the month (1-31).
* * \b HH is the hour number in a 24-hour day (optional, defaults to 0).
* * \b +/-W specifies the day of the week the rule takes effect (where
* Sunday = 0, Monday = 1, etc). +W means that the rule applies
* to the first such day on or after the specified date and -W
* strictly before the date. If this is not specified, the rule
* will take effect on the exact date, regardless of the day of
* the week.
*
* \par Example
* U.S. Eastern Standard time is UTC -5. Eastern Daylight time is UTC -4.
* Daylight time goes into affect on the second sunday of March at 2:00AM local
* time. Daylight time ends on the first sunday of November at 2:00AM local
* time. The law that defines this went into affect in 2007.
* Therefore here is how the DST string is constructed:
* | \| | STD Time | \| | DST Time | \| | UTC Offset | \| | DST Rule Year | Month DST Starts | Day DST Starts | Hour DST Starts | Day of Week | \| | Month DST Ends | Day DST Ends | Hour DST Ends | Day of Week |
* |----|----------|----|----------|----|------------|----|---------------|------------------|----------------|-----------------|-------------|----|----------------|--------------|---------------|-------------|
* | : | XXX | : | YYY | : | NNN | : | (YYYY) | MM | DD* | HH | +/-W** | : | MM | DD | HH | +/-W |
* | : | GMT-5 | : | GMT-4 | : | -0500 | : | (2007) | 03 | 08 | 02 | +0 | : | 11 | 01 | 02 | +0 |
* - * An 8 for the day means that DST will start around the 8th day of the
* month. Or that the +/-W parameter is relative to the 8th day of the month.
* - ** A + here means that the DST rule will start \b on or \b after the
* previously specified day (the 8th). 0 means that it should happen on a
* sunday. Therefore if the 8th is a sunday (and the 8th cannot be the first
* sunday of the month) then the rule will take affect on that day - or it
* will happen on the very next sunday.
* \par
* Result:
* \code
* :GMT-5:GMT-4:-0500:(2007)030802+0:110102+0
* \endcode
*
* \sa
* - time_zones - Supported time zones
*/
const char8_t * __getzone(void)
{
const char8_t *current_zone = NULL;
static const char8_t dst_time_zones[][50] =
{
// UTC time
":GMT+0:GMT+0:0000:0",
// Newfoundland Standard Time UTC – 3:30
":GMT-3:GMT-2:-0330:(2007)030802+0:110102+0",
// Atlantic Standard Time, UTC – 4
":GMT-4:GMT-3:-0400:(2007)030802+0:110102+0",
// Eastern Standard Time, UTC – 5
":GMT-5:GMT-4:-0500:(2007)030802+0:110102+0",
// Central Standard Time, UTC – 6
":GMT-6:GMT-5:-0600:(2007)030802+0:110102+0",
// Mountain Standard Time, UTC – 7
":GMT-7:GMT-6:-0700:(2007)030802+0:110102+0",
// Pacific Standard Time, UTC – 8
":GMT-8:GMT-7:-0800:(2007)030802+0:110102+0",
// Alaska Standard Time, UTC – 9
":GMT-9:GMT-8:-0900:(2007)030802+0:110102+0",
// Hawaii-Aleutian Standard Time, UTC – 10
":GMT-10:GMT-9:-1000:(2007)030802+0:110102+0"
};
static const char8_t std_time_zones[][20] =
{
// UTC time
":GMT+0:GMT+0:0000",
// Newfoundland Standard Time UTC – 3:30
":GMT-3:GMT-2:-0330",
// Atlantic Standard Time, UTC – 4
":GMT-4:GMT-3:-0400",
// Eastern Standard Time, UTC – 5
":GMT-5:GMT-4:-0500",
// Central Standard Time, UTC – 6
":GMT-6:GMT-5:-0600",
// Mountain Standard Time, UTC – 7
":GMT-7:GMT-6:-0700",
// Pacific Standard Time, UTC – 8
":GMT-8:GMT-7:-0800",
// Alaska Standard Time, UTC – 9
":GMT-9:GMT-8:-0900",
// Hawaii-Aleutian Standard Time, UTC – 10
":GMT-10:GMT-9:-1000"
};
switch(get_config()->time_zone)
{
case NST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[NST];
}
else
{
current_zone = std_time_zones[NST];
}
}
break;
case AST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[AST];
}
else
{
current_zone = std_time_zones[AST];
}
}
break;
case EST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[EST];
}
else
{
current_zone = std_time_zones[EST];
}
}
break;
case CST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[CST];
}
else
{
current_zone = std_time_zones[CST];
}
}
break;
case MST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[MST];
}
else
{
current_zone = std_time_zones[MST];
}
}
break;
case PST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[PST];
}
else
{
current_zone = std_time_zones[PST];
}
}
break;
case AKST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[AKST];
}
else
{
current_zone = std_time_zones[AKST];
}
}
break;
case HAST:
{
if(get_config()->b_dst)
{
current_zone = dst_time_zones[HAST];
}
else
{
current_zone = std_time_zones[HAST];
}
}
break;
case UTC:
default:
current_zone = std_time_zones[UTC];
break;
}
return current_zone;
}
答案 1 :(得分：2)
上面的乌克兰统治不太正确，因为它不适用于31日的星期日。如果您阅读getzone.c中的规则，则表示严格遵循此日期之前的星期日。
+/-W specifies the day of the week the rule takes effect (where Sunday = 0, Monday = 1, etc). +W means that the rule applies to the first such day on or after the specified date and -W strictly before the date. If this is not specified, the rule will take effect on the exact date, regardless of the day of the week.
正确的规则如下
char const * __getzone()
{
return ":GMT+2:GMT+3:0200:(1996)032503+0:102504+0";
// Ukraine; rule: (last Sun (March | October) )
}
答案 2 :(得分：1)
char const * __getzone()
{
return ":GMT+2:GMT+3:0200:(1996)033103-0:103104-0";
// Ukraine; rule: (last Sun (March | October) )
}
答案 3 :(得分：1)
对于德国，您可以使用：
char const * __getzone() {
return ":GMT+1:GMT+2:0100:032502+0:102502+0";
}
无论是乌克兰人还是我的，但我用实际设备对我进行了测试并得到了正确的结果。在这里我的测试：
//Forward
time_t ts = 1490489997L;//26.03.2017-01:59:57 3 seconds before dst
struct tm* pre = localtime(&ts);
time_t after = ts + 5L;//wait 5 seconds -> 26.03.2017-03:00:02
struct tm* post = localtime(&after);
//Backward
time_t ts = 1509238797L;//29.10.2017-02:59:57 3 seconds before dst
struct tm* pre = localtime(&ts);
time_t after = ts + 5L;//wait 5 seconds -> 29.10.2017-02:00:02
struct tm* post = localtime(&after);
祝你好运 迈克尔