systemd systemd-timesyncd.service 8 systemd-timesyncd.service systemd-timesyncd Network Time Synchronization systemd-timesyncd.service /usr/lib/systemd/systemd-timesyncd 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.conf5, the per-link static settings in .network files, and the per-link dynamic settings received over DHCP. See systemd.network5 for further details. timedatectl1's set-ntp command may be used to enable and start, or disable and stop this service. timedatectl1'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.special7 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.service8 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. /var/lib/systemd/timesync/clock The modification time ("mtime") of this file is updated on each successful NTP synchronization or after each SaveIntervalSec= time interval, as specified in timesyncd.conf5. When initializing, the local clock is advanced to the modification time of this file (if the file timestamp is in the past this adjustment is not made). If the file does not exist yet, the clock is instead advanced to the modification time of /usr/lib/clock-epoch – if it exists – or to a time derived from the source tree at build time. This mechanism is used to ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across reboots, in case no battery-buffered local RTC is available. /usr/lib/clock-epoch The modification time ("mtime") of this file is used for advancing the system clock in case /var/lib/systemd/timesync/clock does not exist yet, see above. /run/systemd/timesync/synchronized A file that is touched on each successful synchronization, to assist systemd-time-wait-sync and other applications to detecting synchronization with accurate reference clocks. systemd1 timesyncd.conf5 systemd.network5 systemd-networkd.service8 systemd-time-wait-sync.service8 systemd.special7 timedatectl1 localtime5 hwclock8