Many Individual Taxpayer Identification Numbers will expire on Jan. 1, 2017, and ITIN holders hoping to avoid refund delays should renew now, the Internal Revenue Service is warning.

ITINs are used by anyone who has to pay or file U.S. taxes, but isn’t eligible for a Social Security number. Under a recent tax law change, ITINs that haven’t been used on a tax return in the last three years will expire at the beginning of next year.

Also set to expire are ITINs with 78 or 79 as the fourth and fifth digits (9NN-79-NNNN or 9NN-79-NNNN).

While those with expired ITINs can apply for renewal after January 1, the IRS is warning that the usual application period of as little as seven weeks is likely to be as much as four weeks longer for those who submit in January and February.

The service also noted that is seeing a number of common mistakes on renewal applications so far, primarily involving missing information and insufficient supporting information.

To apply, taxpayers should use Form W-7. Help and more information are available at IRS.gov/ITIN.

Daniel Hood is editor-in-chief of Accounting Today and Tax Pro Today, and has covered the tax and accounting field for over 20 years.