difftime 在特定日期返回奇怪的值

问题描述：

我不明白为什么 difftime 返回一个奇怪的值，所以这里是数据集和我使用的代码.

I don't understand why difftime returns a strange value, so here is the dataset and the code I am using.

代码:

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;
currentTime.tm_wday = 7 - 1;

struct tm previousTime;
previousTime.tm_year = 2014 - 1900;
previousTime.tm_mon = 9 - 1;
previousTime.tm_mday = 6;
previousTime.tm_hour = 23;
previousTime.tm_min = 58;
previousTime.tm_sec = 0;
previousTime.tm_wday = 7 - 1;

cout << difftime(mktime(&currentTime), mktime(&previousTime)) << endl;

这会打印:

任何想法为什么我得到这个值?我应该得到 60，因为有一分钟的差异.我尝试了其他一些值，它们都有效..我正在使用带有 mingw 的 CodeBlocks.

Any ideas why I get this value ? I should get 60, as there is a one minute difference. I tried with some other values, and they all work.. I'm using CodeBlocks with mingw.

答案:使用 tm_isdst 解决了问题！血腥夏令时:P

EDIT : Answer : use of tm_isdst solved the problem ! Bloody DST :P

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;
currentTime.tm_wday = 7 - 1;
currentTime.tm_isdst = - 1;

struct tm previousTime;
previousTime.tm_year = 2014 - 1900;
previousTime.tm_mon = 9 - 1;
previousTime.tm_mday = 6;
previousTime.tm_hour = 23;
previousTime.tm_min = 58;
previousTime.tm_sec = 0;
previousTime.tm_wday = 7 - 1;
previousTime.tm_isdst = - 1;

cout << difftime(mktime(&currentTime), mktime(&previousTime)) << endl;

答

在调用 mktime() 之前，通常需要设置 struct tm 的 7 个字段.由于 OP 只设置了其中的 6 个，未初始化的数据位于 tm_isdst 字段中，导致 3600 秒的意外偏移.

Before calling mktime(), typically 7 fields of struct tm need to be set. As OP only set 6 of those, uninitialized data was in field tm_isdst causing an unexpected shift of 3600 seconds.

struct tm currentTime;
currentTime.tm_year = 2014 - 1900;
currentTime.tm_mon = 9 - 1;
currentTime.tm_mday = 6;
currentTime.tm_hour = 23;
currentTime.tm_min = 59;
currentTime.tm_sec = 0;

currentTime.tm_isdst = -1;  // **
// currentTime.tm_wday = 7 - 1; 

mktime(&currentTime);

建议零填充 struct tm 如 struct tm currentTime = { 0 }; 以确保所有字段都指定为 struct tm可能包含除 9 之外的字段:int tm_sec tm_min tm_hour tm_mday tm_mon tm_year tm_wday tm_yday tm_isdst.

Recommend to zero-fill struct tm as in struct tm currentTime = { 0 }; to insure all fields are specified as struct tm may contain fields in addition to the 9: int tm_sec tm_min tm_hour tm_mday tm_mon tm_year tm_wday tm_yday tm_isdst.

注意事项:

tm_wday 和 tm_yday 中的原始值被 mktime() 忽略并重新计算.其他字段的原始值不受其正常范围的限制，也会重新计算.

The original values in tm_wday and tm_yday are ignored and re-calculated by mktime(). The other fields' original values are not restricted to their normal range and are recalculated too.

** tm_isdst 的正值或零值会导致 mktime 函数最初分别假定夏令时在指定时间内有效或无效.负值会导致它尝试确定夏令时是否在指定时间内生效.

** a positive or zero value for tm_isdst causes the mktime function to presume initially that Daylight Saving Time, respectively, is or is not in effect for the specified time. A negative value causes it to attempt to determine whether Daylight Saving Time is in effect for the specified time.

difftime 在特定日期返回奇怪的值

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