All crew are also under a regulation that limits their time available for flight, similar to a trucker but with larger consequences. eg If you are on shift for a certain number of hours, waiting for a plane in a hotel or in the terminal, it_does_not_matter. You have to have a mandatory sleep block where you are now unavailable. Once thousands of staff started timing out, even after the weather clears, it_did_not_matter. They were now unavailable and their replacements hadn't come in due to the weather, leaving some locales understaffed and all schedules backed up (which continued to compound as weather shifted).