systemd-timesyncd is a system service that may be used to synchronize the local system clock with a remote Network Time Protocol (NTP) server. It also saves the local time to disk every time the clock has been synchronized and uses this to possibly advance the system realtime clock on subsequent reboots to ensure it (roughly) monotonically advances even if the system lacks a battery-buffered RTC chip.
The systemd-timesyncd service implements SNTP only. This minimalistic service will step the system clock for large offsets or slowly adjust it for smaller deltas. Complex use cases that require full NTP support (and where SNTP is not sufficient) are not covered by systemd-timesyncd.
The NTP servers contacted are determined from the global settings in timesyncd.conf(5), the per-link static settings in .network files, and the per-link dynamic settings received over DHCP. See systemd.network(5) for further details.
timedatectl(1)'s set-ntp command may be used to enable and start, or disable and stop this service.
timedatectl(1)'s timesync-status or show-timesync command can be used to show the current status of this service.
systemd-timesyncd initialization delays the start of units that are ordered after time-set.target (see systemd.special(7) for details) until the local time has been updated from /var/lib/systemd/timesync/clock (see below) in order to make it roughly monotonic. It does not delay other units until synchronization with an accurate reference time sources has been reached. Use systemd-time-wait-sync.service(8) to achieve that, which will delay start of units that are ordered after time-sync.target until synchronization to an accurate reference clock is reached.