As an aside: the V2 stack is open source 
https://github.com/TheThingsNetwork/ttn/blob/v2.10.0/core/networkserver/uplink_mac.go shows nothing that relates to this. Of course, it could be hidden elsewhere, but as this uses https://github.com/brocaar/lorawan/blob/master/mac_commands.go, one can peek into that code to reveal the name would be just as expected:
DeviceTimeReq CID = 0x0D
DeviceTimeAns CID = 0x0D
Hence, it’s safe to assume that a search for DeviceTimeReq in the ttn repository would return some results, if implemented.
That said, @htdvisser, the V3 stack is still closed, right?