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Extract of FreeBSD manpage of clock_gettime:
 The clock_id argument can be one of the following values: CLOCK_REALTIME,
 CLOCK_REALTIME_PRECISE, CLOCK_REALTIME_FAST for time that increments as a
 wall clock should; CLOCK_MONOTONIC, CLOCK_MONOTONIC_PRECISE,
 CLOCK_MONOTONIC_FAST which increments in SI seconds; CLOCK_UPTIME,
 CLOCK_UPTIME_PRECISE, CLOCK_UPTIME_FAST which starts at zero when the
 kernel boots and increments monotonically in SI seconds while the machine
 is running; CLOCK_VIRTUAL for time that increments only when the CPU is
 running in user mode on behalf of the calling process; CLOCK_PROF for
 time that increments when the CPU is running in user or kernel mode; or
 CLOCK_SECOND which returns the current second without performing a full
 time counter query, using in-kernel cached value of current second.
 The clock IDs CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST,
 CLOCK_UPTIME_FAST are analogs of corresponding IDs without _FAST suffix
 but do not perform a full time counter query, so their accuracy is one
 timer tick. Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE,
 CLOCK_UPTIME_PRECISE are used to get the most exact value as possible, at
 the expense of execution time.