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. 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
For an explanation of the terms used in this section, see attributes(7).
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
On a POSIX system, time_t 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
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