The easiest way to handle leap seconds is with the POSIX time rules FreeBSD uses by default, combined with NTP. When ntpd(8) is running and the time is synchronized with upstream NTP servers that handle leap seconds correctly, the leap second will cause the system time to automatically repeat the last second of the day. No other adjustments are necessary.

If the upstream NTP servers do not handle leap seconds correctly, ntpd(8) will step the time by one second after the errant upstream server has noticed and stepped itself.

If NTP is not being used, manual adjustment of the system clock will be required after the leap second has passed.