Os.MtimeSourceMonotonic time clock and sleep.
See B0_std.Mtime for a discussion about monotonic time.
now () is the current system-relative monotonic timestamp. Its absolute value is meaningless.
elapsed () is the monotonic time span elapsed since the beginning of the program.
The type for monotonic wall-clock time counters.
count c is the monotonic time span elapsed since c was created.
The functions elapsed, now, counter, raise Sys_error whenever they can't determine the current time or that it doesn't fit in Mtime's range. Usually this exception should only be catched at the toplevel of your program to log it and abort the program. It indicates a serious error condition in the system.
clock_gettime with CLOCK_MONOTONIC.mach_absolute_time.