[ttnctl-user] Use UTC time for expiration
[ttnctl-user] Use UTC time for expiration

TheThingsNetwork/ttn,TheThingsNetwork/ttn,LoRaWanSoFa/ttn,mgranberry/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,mgranberry/ttn,jvanmalder/ttn,mgranberry/ttn,jvanmalder/ttn,LoRaWanSoFa/ttn