It seems you could get close to ensuring such a time would exist by creating constant emergencies for the crew to deal with such that rebooting would be hazardous, and waiting for the reboot-or-crash date to launch the major attack.
What would seal the deal would be if there was some external signal that revealed that the crash happened. This could be as easy to arrange as a confederate onboard with a powerful laser pointer.
It seems you could get close to ensuring such a time would exist by creating constant emergencies for the crew to deal with such that rebooting would be hazardous, and waiting for the reboot-or-crash date to launch the major attack.