Back in the spring, millions of Americans asked for and received six additional months to file their federal income tax returns.

Now, the extension is almost up. The new deadline is Tuesday.

“Time is running out,” said Lance Christensen, a tax partner at Margolin, Winer & Evens in Garden City, N.Y. “They have to file.”

A record 15 million people — about one in 10 filers — received automatic, six-month filing extensions for their 2018 returns, according to the Internal Revenue Service. (That was up from 14.7 million the year before.) Sweeping changes to the tax code under the Tax Cuts and Jobs Act of 2017 were behind the torrent of extensions, according to tax professionals and the I.R.S.

Missing the extended deadline could mean penalties from the I.R.S. for failing to file and — if the filer still owes taxes — failure to pay. The extension allowed an extra six months to file a return but no extra time to pay. (Late filers are supposed to estimate what they owe and pay by the original April deadline.) So penalties may already be accruing for those who haven’t paid, and will grow if the October deadline is missed, too.