Unfortunately, there is no guaranteed way to do this - you just cannot rely upon the network:
https://forum.sierrawireless.com/t/date-time-time-zone-information/3962/2
https://forum.sierrawireless.com/t/get-current-date-time-from-network/739/10
https://forum.sierrawireless.com/t/getting-network-date-and-time/2812/4
https://forum.sierrawireless.com/t/date-and-time-in-q2686/4701/1
Note that this is a network limitation - nothing specifically to do with SiWi