systemd-timedated.service
/usr/lib/systemd/systemd-timedated
systemd-timedated.service is a system service that may be used as a mechanism to change the system clock and timezone, as well as to enable/disable network time synchronization. systemd-timedated is automatically activated on request and terminates itself when it is unused.
The tool timedatectl(1) is a command line client to this service.
systemd-timedated currently offers access to the following four settings:
See org.freedesktop.timedate1(5) and org.freedesktop.LogControl1(5) for information about the D-Bus API.
systemd-timesyncd will look for files with a ".list" extension in ntp-units.d/ directories. Each file is parsed as a list of unit names, one per line. Empty lines and lines with comments ("#") are ignored. Files are read from /usr/lib/systemd/ntp-units.d/ and the corresponding directories under /etc/, /run/, /usr/local/lib/. Files in /etc/ override files with the same name in /run/, /usr/local/lib/, and /usr/lib/. Files in /run/ override files with the same name under /usr/. Packages should install their configuration files in /usr/lib/ (distribution packages) or /usr/local/lib/ (local installs).
Example 1. ntp-units.d/ entry for systemd-timesyncd
# /usr/lib/systemd/ntp-units.d/80-systemd-timesync.list systemd-timesyncd.service
If the environment variable $SYSTEMD_TIMEDATED_NTP_SERVICES is set, systemd-timesyncd will parse the contents of that variable as a colon-separated list of unit names. When set, this variable overrides the file-based list described above.
Example 2. An override that specifies that chronyd should be used if available
SYSTEMD_TIMEDATED_NTP_SERVICES=chronyd.service:systemd-timesyncd.service
systemd(1), timedatectl(1), localtime(5), hwclock(8), systemd-timesyncd(8)