Some sandboxing could be done, but if an attacker roots a pacemaker or an insulin pump it will be extremely difficult if not impossible to prevent them from convincing the device to perform its intended function at an unintended time.

Airgapped. Not just sandboxed. As in two separate processors, with no overlapping RAM / etc.

You can't do your example, because there's no way to get at the internal clock from the radio.