如何正确地从tm转换为time_t？

Question

I want to create a tm and fill it with a specific time, then I want to subtract a certain amount of time from it.

This is what I have so far

time_t time_1, time_2;
struct tm tm;
struct tm tm2;

strptime("1 1 1900 12:43:40", "%d %m %Y %H:%M:%S", &tm);
strptime("1 1 1900 11:33:45", "%d %m %Y %H:%M:%S", &tm2);


time_1 = mktime(&tm);
time_2 = mktime(&tm2);

//    time_1  = difftime(time_2,  time_1);                                                
cout << ctime(&time_1) << endl;
cout << asctime(&tm);

I was planning on using difftime to calculate the difference, but ctime isn't displaying the same time as asctime, where am I going wrong?

Answer 1

The difference is likely to be due to daylight saving time differences - ie the tm_isdst field of the struct tm. The call ctime(t) is equivalent to asctime(localtime(t)), so try calling just localtime(time_1) and compare the result, field by field, to tm. I would also advise not calling your variable tm as it can be confusing when it's the same name as the struct, especially as this is C++ and therefore the struct keyword is optional.