difftime - calculate time difference
#include <time.h> double difftime(time_t time1, time_t time0);
The difftime() function returns the number of seconds elapsed between time time1 and time time0. The two times are specified in calendar time, which represents the time elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time (UTC).
SVID 3, BSD 4.3, ISO 9899
date(1), gettimeofday(2), time(2), ctime(3), gmtime(3), localtime(3)