function returns the number of seconds elapsed
between time time1 and time time0, represented as a
Each of the times is specified in calendar time, which means its
value is a measurement (in seconds) relative to the
Epoch, 1970-01-01 00:00:00 +0000 (UTC).
SVr4, 4.3BSD, C89, C99.
On a POSIX system,
is an arithmetic type, and one could just
#define difftime(t1,t0) (double)(t1 - t0)
when the possible overflow in the subtraction is not a concern.
On other systems, the data type
might use some other encoding
where subtraction doesn't work directly.