"double difftime(time_t "time1", time_t "time0);
The difftime() function returns the number of seconds elapsed between time time1 and time time0, represented as a double. The two times are specified in calendar time, which represents the time elapsed since the Epoch (00:00:00 on January 1, 1970, Coordinated Universal Time (UTC)).
SVID 3, BSD 4.3, ISO 9899
This function is required by ANSI C. On a POSIX system, time_t is an arithmetic type, and one could just define
#define difftime(t1,t0) (double)(t1 - t0)
when the possible overflow in the subtraction is not a concern.
date(1), gettimeofday(2), time(2), ctime(3), gmtime(3), localtime(3)