Honestly, time-keeping doesn't seem that difficult. It does not have to derive from GPS satellites - there are highly accurate clocks earthside. Any system requiring highly accurate time should be able to access an earthside time service.
Jamming GPS as a positioning systems seems a larger problem: lots of systems depend on this. Consider the huge agricultural machines run their routes based on GPS. Consider ocean-going freighters. Consider airline autopilots. All of these systems would require weeks to months to work around a systemic failure of GPS.
