The documentation at http://www.freedesktop.org/software/systemd/man/systemd-timesyncd.service.html says:
> This file contains the timestamp of last successful synchronization.
However, that file is created even if systemd-timesyncd couldn't connect to a server. See src/timesync/timesyncd.c, lines 63-64:
> /* create stamp file with the compiled-in date */
> touch_file("/var/lib/systemd/clock", true, min, uid, gid, 0644);
I wouldn't call creating a file with the timestamp of the systemd build a "successful synchronization".