From cppreference.com
< c‎ | chrono
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.


time_beg, time_end - times to compare

Return value

Difference between two times in seconds.


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.


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;
    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;


1937968 seconds have passed since the beginning of the month.


  • C11 standard (ISO/IEC 9899:2011):
  • The difftime function (p: 390)
  • C99 standard (ISO/IEC 9899:1999):
  • The difftime function (p: 339)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • The difftime function

See also

C++ documentation for difftime