systemd-time-wait-sync is a system service that delays the start of units that depend on time-sync.target until the system time has been synchronized with an accurate time source by systemd-timesyncd.service.
systemd-timesyncd.service notifies on successful synchronization. systemd-time-wait-sync also tries to detect when the kernel marks the time as synchronized, but this detection is not reliable and is intended only as a fallback for other services that can be used to synchronize time (e.g., ntpd, chronyd).
The presence of this file indicates to this service that the system clock has been synchronized.
systemd(1), systemd.special(7), systemd-timesyncd.service(8),
systemd.directives(7), systemd.index(7), systemd-timesyncd.service(8).
The man page systemd-time-wait-sync(8) is an alias of systemd-time-wait-sync.service(8).