A GPS receiver would do the trick. The difficulty would be in verifiably making sure the GPS location doesn’t leave the phone once established (as far as I know, it doesn’t go back out through GPS). Which, presumably, the kill switches would accomplish…though someone will no doubt (please do!) correct me if I am wrong.
If the phone has a charged battery (and it needs, probably, at least 5 percent), it should keep time fairly well without needing a sync. Mine has since the initial issues.