在Windows中手动更改UTC时区时，time函数将返回之前时区值的相同值（即time_t secs）

Question

Step:1 Set the time zone to (UTC-08:00) Pacific Time (US & Canada) in windows.

Step:2 Checked the value of time(&secs) using a C Program, I got secs = 1386043600 .

Step:3 Changed the time zone to (UTC+05:30) Chennai, Kolkata, Mumbai, New Delhi manually.

Step:4 Checked the value of time(&secs). Again I got the same value, secs = 1386043600 .

After this I have rebooted the machine and again checked the value of time(&secs) . Now the value has changed to secs = 1386046505. Does this value gets effected only after rebooting the system?

Answer 1

This is expected behavior. A time_t value is the number of seconds since midnight (GMT) on 1 January 1970 — as if every computer had a counter counting up by 1 every second from then, and the value is what time() returns. It is intentionally independent of the time zone your computer is set to use.

Answer 2

Trying running this sample code

#include<stdio.h>
#include<stdlib.h>
#include<time.h>
#include<windows.h>

int main(int argc, char* argv[])
{
    time_t t,prev_t;
    int index=1;
    SYSTEMTIME st, lt;
    printf("\n\tSetting the Time Zone to \"Central Standard Time\"");
    system("RunDLL32.exe shell32.dll,Control_RunDLL timedate.cpl,,/Z Central Standard Time");

    prev_t = time(NULL) ;
    printf("\nStarting Timetick %d", prev_t);
    while (index<=9) 
     {

        if( index==5)
        {
            printf("\n\tSetting the Time Zone to \"Eastern Standard Time\"");
            system("RunDLL32.exe shell32.dll,Control_RunDLL timedate.cpl,,/Z Eastern Standard Time");
        }
        memset(&st,'\0',sizeof(st));
        memset(&lt,'\0',sizeof(lt));

        Sleep(1000);
        t=time(NULL);
        GetSystemTime(&st); /* UTC */
        GetLocalTime(&lt);  /* local */

        printf("\n[%d] system(utc): %02d:%02d:%02d, local: %02d:%02d:%02d, time()=%ld (diff=%ld)", index,st.wHour, st.wMinute,st.wSecond,lt.wHour,lt.wMinute,lt.wSecond,t, t- prev_t);

        prev_t=t;
        index++;

    }
}

1) At startup, sample sets the current Time Zone to "Central Standard Time". And, also prints the current time tick as returned by time().

2) Then it executes a loop 9 times. Every iteration of the loop finds UTC time, localtime and time ticks using function GetSystemTime(), GetLocalTime() and time() respectively. These values are printed along with the difference in the previous time tick.

3) At 5th iteration, the Time Zone is changed to "Eastern Standard Time".

4) Every iteration is executed with a time gap of one second, using function Sleep().

5) The Time Zone change in the sample is achieved with the function system().

This out put of the code

Setting the Time Zone to "Central Standard Time"
Starting Timetick 1385974984
[1] system(utc): 09:03:05, local: 14:33:05, time()=1385974985 (diff=1)
[2] system(utc): 09:03:06, local: 14:33:06, time()=1385974986 (diff=1)
[3] system(utc): 09:03:07, local: 14:33:07, time()=1385974987 (diff=1)
[4] system(utc): 09:03:08, local: 14:33:08, time()=1385974988 (diff=1)
        Setting the Time Zone to "Eastern Standard Time"
[5] system(utc): 09:03:12, local: 14:33:12, time()=1385974992 (diff=4)
[6] system(utc): 09:03:13, local: 14:33:13, time()=1385974993 (diff=1)
[7] system(utc): 09:03:14, local: 14:33:14, time()=1385974994 (diff=1)
[8] system(utc): 09:03:15, local: 14:33:15, time()=1385974995 (diff=1)
[9] system(utc): 09:03:16, local: 14:33:16, time()=1385974996 (diff=1)
Process returned 1385974996 (0x529C4CD4)   execution time : 13.397 s
Press any key to continue.