Add the half of time-of-day clock resolution when we adjust system time from time-of-day clock or vice versa.  For x86 systems, RTC resolution is one second and we used to lose up to one second whenever we initialize system time from RTC or write system time back to RTC.  With this change, margin of error per conversion is roughly between -0.5 and +0.5 second rather than between -1 and 0 second.  Note that it does not take care of errors from getnanotime(9) (which is up to 1/hz second) or CLOCK_GETTIME() latency. These are just too expensive to correct and it is not worthy of the cost.
Add the half of time-of-day clock resolution when we adjust system time from
time-of-day clock or vice versa.  For x86 systems, RTC resolution is one
second and we used to lose up to one second whenever we initialize system
time from RTC or write system time back to RTC.  With this change, margin
of error per conversion is roughly between -0.5 and +0.5 second rather
than between -1 and 0 second.  Note that it does not take care of errors
from getnanotime(9) (which is up to 1/hz second) or CLOCK_GETTIME() latency.
These are just too expensive to correct and it is not worthy of the cost.

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