There is no good time to find out your engine won't operate because of contaminated diesel fuel.

A sudden need for diesel fuel that has been stored to run back up systems and generators can create another disaster after the first 3 to 5 minutes. Those systems usually run until stagnant fuel clogs and "kills" engines.

Boats and ships can be stranded until the filters are changed (if they have some with them). Worse; life support and evacuation systems can quit in commercial buildings, financial institutions, hospitals and communication facilities. Since no one can predict an emergency, the only safe method is to stop the problem before it starts.