I suggest you get time from GPS device.
Many OEM GPS modules have battery & RTC on board (access via serial port - secondary).
GPS device is capable to get time from sky too, of course, when there are visible satellites.
OEM modules have some additional capabilities (at least devices with ARM MCU, that i have). They have 2 serial ports, one of them can be accessed from additional s/w inside module (to write this s/w you must have dveloper kit for module).
There is RTC onboard also, that can be accessed in similar way.
If your GPS modules don't have user-programming capabilities, the only solution is to use external RTC chip with serial interface and built-in battery, that will be synchronized from GPS data.
At this moment this project is in final testing stage.
We plan to build some working real devices in september.
Tis is : N12 with full battery backup (internal) + GPS module with external active GPS antenna (of course, with RTC).
Mainly for automotive use.
Our basic products are meter reading & remote control systems.