As the April 15 filing deadline looms, the Internal Revenue Service reminds taxpayers that after three years, any unclaimed 2021 tax year refunds go to the U.S. Treasury.

More than a million American taxpayers could be leaving cash on the table as the deadline to claim tax refunds from three years ago is fast approaching. The Internal Revenue Service (IRS) has issued a warning that more than a million taxpayers are still eligible to receive a share of more than $1 billion in refunds—but they need to act quickly.

“Under the law, taxpayers usually have three years to file and claim their tax refunds. If they don’t file within three years, the money becomes the property of the U.S. Treasury,” according to the IRS.