For future reference, see the LoRaWAN Application Layer Clock Synchronization Specification v1.0.0:
This document proposes an application layer messaging package running over LoRaWAN to synchronize the real-time clock of an end-device to the network’s GPS clock with second accuracy.
Note:
An end-device using LoRaWAN 1.1 or above SHOULD use DeviceTimeReq MAC command instead of this package. […] End-devices with an accurate external clock source (e.g.: GPS) SHOULD use that clock source instead.
(With contributions from @johan, it seems.)