difftime - calculate time difference
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
when the possible overflow in the subtraction is not a concern.
#define difftime(t1,t0) (double)(t1 - t0)