CODEWITHSUNDEEP
c
user5431949
user5431949

Reputation:

How to check datetime in C

I have the following code:

int main(void) {
        struct tm str_time;
        time_t time_of_day;

        str_time.tm_year = 2012-1900;
        str_time.tm_mon = 6;
        str_time.tm_mday = 5;
        str_time.tm_hour = 10;
        str_time.tm_min = 3;
        str_time.tm_sec = 5;
        str_time.tm_isdst = 0;

        time_of_day = mktime(&str_time);
        printf(ctime(&time_of_day));

        return 0;
}

It works perfectly, but I can't find a way to verify that the date and time are in the code is the same as the computer, does anyone have any idea of how to compare both dates?

Upvotes: 1

Views: 379

Answers (1)

chux
chux

Reputation: 153498

... have any idea of how to compare both dates?

OP is doing 3 things that contribute to potential time differences.

  1. Code may not have filled all needed fields before calling mktime(&str_time);. C specifies at least 9 fields. Best to zero-fill str_time and then set the 7 fields. This is relatively a rare problem.

  2. OP comments about date "2016-06-25 06:58:31". Yet does not post the values use to populate struct tm. A common mis-code is that tm_mon is the months since January, so a minus 1 is needed.

    str_time.tm_year = 2016-1900;
str_time.tm_mon = 6 - 1;
str_time.tm_mday = 25;
str_time.tm_hour = 6;
str_time.tm_min = 58;
str_time.tm_sec = 31;
str_time.tm_isdst = tbd; // see below

  3. str_time.tm_isdst = 0; sets the timestamp to no daylight-savings-time. Often it is better to use str_time.tm_isdst = -1; and let the system determine if DST was in effect.

Rather than compute the time_t for a given user year-month-day, recommend to take the computer's time and convert that to year-month-day and then compare. Certainly any differences / discrepancies will be easier to understand.

To compare the computer local year-moth-day to user input

// February 20, 2016
int y = 2016;
int m = 2;
int d = 20;

time_t now;
if (time(&now) == -1) Handle_Error();
struct tm *tm = localtime(&now);
if (tm == NULL) Handle_Error();

if (((tm->tm_year + 1900) == y) && ((tm->tm_mon + 1) == m) && (tm->tm_mday == d)) {
  puts("Dates match");
}

Upvotes: 2

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