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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 (UTC).
SVr4, 4.3BSD, C89, C99.
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 concern. On
other systems, the data type time_t might use some other encoding where
subtraction doesn't work directly.
date(1), gettimeofday(2), time(2), ctime(3), gmtime(3), localtime(3)
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