Ah! The internal clock relies on GPS to set itself, so if you’re inside from power on, we don’t know the date, therefore can’t calculate the days until expiry. (This is one of the unintended side effects of me testing with an external antenna at the bench!)
From a user’s perspective, what do you think we should do? Print a “No GPS” if GPS is unavailable, and "Unknown’ for other faults? -1 is obviously pretty unhelpful.