The system clock is read and set with microsecond resolution, the precision is limited mostly by D-Bus latency. When setting the clock to an absolute time, a correction is applied for the time spent in the polkit authorization check. The clock can be also adjusted precisely by an offset.
The RTC is read with a resolution of one second. The hwclock program is used to set the RTC with a high precision, update the estimated drift, and change the UTC/LOCAL setting.
The system timezone is set by creating symlink /etc/localtime to a file in the /usr/share/zoneinfo directory.
NTP units that should be controlled by timedatex need to be listed in text files in /etc/systemd/ntp-units.d and /usr/lib/systemd/ntp-units.d directories. The first valid unit from the files sorted by name can be enabled/disabled and started/stopped. Other NTP units will be disabled and stopped.