difftime

Defined in header `<time.h>`
double difftime( time_t time_end, time_t time_beg );

Computes difference between two calendar times as time_t objects (time_end - time_beg) in seconds. If time_end refers to time point before time_beg then the result is negative.

Parameters

time_beg, time_end

times to compare

Return value

Difference between two times in seconds.

Notes

On POSIX systems, time_t is measured in seconds, and difftime is equivalent to arithmetic subtraction, but C and C++ allow fractional units for time_t.

Example

The following program computes the number of seconds that have passed since the beginning of the month.

#include <stdio.h>
#include <time.h>
 
int main(void)
{
    time_t now;
    time(&now);
 
    struct tm beg;
    beg = *localtime(&now);
 
    // set beg to the beginning of the month
    beg.tm_hour = 0;
    beg.tm_min = 0;
    beg.tm_sec = 0;
    beg.tm_mday = 1;
 
    double seconds = difftime(now, mktime(&beg));
 
    printf("%.f seconds have passed since the beginning of the month.\n", seconds);
 
    return 0;
}

Output:

1937968 seconds have passed since the beginning of the month.

References

C11 standard (ISO/IEC 9899:2011):

7.27.2.2 The difftime function (p: 390)

C99 standard (ISO/IEC 9899:1999):

7.23.2.2 The difftime function (p: 339)

C89/C90 standard (ISO/IEC 9899:1990):

4.12.2.2 The difftime function