Sign-up season for President Barack Obama’s health care law doesn’t start for another couple of months, but the next few days are crucial for hundreds of thousands of customers at risk of losing financial aid when they renew coverage for 2016. Call them tardy tax filers: an estimated 1.8 million households that got subsidies for their premiums last year but failed to file a 2014 tax return as required by the law, or left out key IRS paperwork.