One way to represent an elapsed time is with a simple arithmetic data
type, as with the following function to compute the elapsed time between
two calendar times. This function is declared in time.h.

The difftime function returns the number of seconds of elapsed
time between calendar time time1 and calendar time time0, as
a value of type double. The difference ignores leap seconds
unless leap second support is enabled.

In the GNU system, you can simply subtract time_t values. But on
other systems, the time_t data type might use some other encoding
where subtraction doesn't work directly.

The GNU C library provides two data types specifically for representing
an elapsed time. They are used by various GNU C library functions, and
you can use them for your own purposes too. They're exactly the same
except that one has a resolution in microseconds, and the other, newer
one, is in nanoseconds.

— Data Type: struct timeval

The struct timeval structure represents an elapsed time. It is
declared in sys/time.h and has the following members:

long int tv_sec

This represents the number of whole seconds of elapsed time.

long int tv_usec

This is the rest of the elapsed time (a fraction of a second),
represented as the number of microseconds. It is always less than one
million.

— Data Type: struct timespec

The struct timespec structure represents an elapsed time. It is
declared in time.h and has the following members:

long int tv_sec

This represents the number of whole seconds of elapsed time.

long int tv_nsec

This is the rest of the elapsed time (a fraction of a second),
represented as the number of nanoseconds. It is always less than one
billion.

It is often necessary to subtract two values of type struct timeval or struct timespec. Here is the best way to do
this. It works even on some peculiar operating systems where the
tv_sec member has an unsigned type.

/* Subtract the `struct timeval' values X and Y,storing the result in RESULT.Return 1 if the difference is negative, otherwise 0. */
int
timeval_subtract (result, x, y)
struct timeval *result, *x, *y;
{
/* Perform the carry for the later subtraction by updating y. */
if (x->tv_usec < y->tv_usec) {
int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
y->tv_usec -= 1000000 * nsec;
y->tv_sec += nsec;
}
if (x->tv_usec - y->tv_usec > 1000000) {
int nsec = (x->tv_usec - y->tv_usec) / 1000000;
y->tv_usec += 1000000 * nsec;
y->tv_sec -= nsec;
}
/* Compute the time remaining to wait.tv_usec is certainly positive. */
result->tv_sec = x->tv_sec - y->tv_sec;
result->tv_usec = x->tv_usec - y->tv_usec;
/* Return 1 if result is negative. */
return x->tv_sec < y->tv_sec;
}

Common functions that use struct timeval are gettimeofday
and settimeofday.

There are no GNU C library functions specifically oriented toward
dealing with elapsed times, but the calendar time, processor time, and
alarm and sleeping functions have a lot to do with them.

Published under the terms of the GNU General Public License