why gmtime and localtime give me the same result?

Question

I have following test code to see the difference between gmtime and localtime. But they give me the same result: UTC:2013-05-02T13:59:58 Local:2013-05-02T13:59:58

time_t now;
time(&now);
tm *pTimeStruct = gmtime(&now);
tm *plocalTimeStruct = localtime(&now);

string timeStr = "";
char timeBuf[64] = {'\0'};

sprintf(timeBuf,"UTC:%-4.4d-%-2.2d-%-2.2dT%-2.2d:%-2.2d:%-2.2d "
    "Local:%-4.4d-%-2.2d-%-2.2dT%-2.2d:%-2.2d:%-2.2d",
    (pTimeStruct->tm_year + 1900),
    (pTimeStruct->tm_mon + 1),
    pTimeStruct->tm_mday,
    pTimeStruct->tm_hour,
    pTimeStruct->tm_min,
    pTimeStruct->tm_sec,
    (plocalTimeStruct->tm_year + 1900),
    (plocalTimeStruct->tm_mon + 1),
    plocalTimeStruct->tm_mday,
    plocalTimeStruct->tm_hour,
    plocalTimeStruct->tm_min,
    plocalTimeStruct->tm_sec);

timeStr += timeBuf;
cout << timeStr << endl;

EDIT:

I am in Eastern time zone.

EDIT2:

updated code use diff struct, but got the same result:

        time_t now;
        time(&now);
        time_t now2;
        time(&now2);
        tm *pTimeStruct = gmtime(&now);
        tm *plocalTimeStruct = localtime(&now2);

Answer 1

You need to copy the values between the calls to gmtime and localtime:

The return value points to a statically allocated struct which might be overwritten by subsequent calls to any of the date and time functions.

says the man page on my system. That's common behaviour at least on Linux.

Answer 2

You can also use gmtime_r and localtime_r, which are thread safe and store the data in a user-supplied struct.

struct tm *gmtime_r(const time_t *timep, struct tm *result);
struct tm *localtime_r(const time_t *timep, struct tm *result);

Note for Windows users: _gmtime_s and _localtime_s are the Microsoft versions.

errno_t _gmtime_s(struct tm* _tm, const __time_t* time);
errno_t _localtime_s(struct tm* _tm, const time_t *time);

Answer 3

I also had this problem and solved it by using memcpy:

time_t t = time(NULL);  

tm* gmt = (tm*)malloc(sizeof(tm));
memcpy(gmt, gmtime(&t), sizeof(tm));

tm* loc = localtime(&t);

cout << asctime(gmt) << endl;
cout << asctime(loc) << endl;   

free(gmt);