Confirmed, that time kept well on newer resinOS devices, just tried it on a 2.2.0+rev1 Raspberry Pi 3, and the host OS logs show this after a reboot:
Jul 28 05:23:00 raspberrypi3 systemd-timesyncd[560]: System clock time unset or jumped backwards, restoring from recorded timestamp: Fri 2017-08-04 10:16:53 UTC
Aug 04 10:16:53 raspberrypi3 systemd[1]: Time has been changed
Aug 04 10:16:53 raspberrypi3 systemd[1]: Started Network Time Synchronization.
And as mentioned, this is available since resinOS 2.0.3. If you have any devices to update to newer version, we are beta-testing updates, see more in the announcement 