When Should You File Form CT-1 X?

File Form CT-1 X when you discover an error on a previously filed Form CT-1.

However, if your only errors on Form CT-1 relate to RRTA tax liabilities reported in Part II of Form CT-1 or on Form 945-A, Annual Record of Federal Tax Liability, don’t file Form CT-1 X. To correct federal tax liabilities reported in Part II of Form CT-1, file Form 945-A, but only enter the monthly totals. The daily entries aren’t required. For more information about correcting RRTA tax liabilities reported on Form 945-A, see the Instructions for Form 945-A.

Due dates. The due date for filing Form CT-1 X depends on when you discover an error and if you underreported or

Instructions for Form CT-1 X (Rev. 4-2025) 3

overreported tax. If you underreported tax, see Underreported tax, later. For overreported tax amounts, you may choose to either make an interest-free adjustment or file a claim for refund or abatement. If you’re correcting overreported tax amounts, see Overreported tax—Adjustment process or Overreported tax—Claim process , later.

If any due date falls on a Saturday, Sunday, or legal holiday, you may file Form CT-1 X on the next business day. The term “legal holiday” means any legal holiday in the District of Columbia. See Pub. 15 for the list of legal holidays. If we receive Form CT-1 X after the due date, we will treat Form CT-1 X as filed on time if the envelope containing Form CT-1 X is properly addressed, contains sufficient postage, and is postmarked by the U.S. Postal Service (USPS) on or before the due date, or sent by an IRS-designated private delivery service (PDS) on or before the due date. If you don’t follow these guidelines, we will consider Form CT-1 X filed when it is actually received. For more information about PDSs, see Where Should You File Form CT-1 X, later.

Underreported tax. If you’re correcting underreported tax, you must file Form CT-1 X by the due date of the return for the return period in which you discovered the error and pay the amount you owe by the time you file . Filing and paying on time generally will ensure that your correction is interest free and not subject to failure-to-pay (FTP) or failure-to-deposit (FTD) penalties. See What About Penalties and Interest, later. For details on how to make a payment, see the instructions for line 26, later.

If Form CT-1 X is filed late (after the due date of the return for the return period in which you discovered the error), you must attach an amended Form 945-A to Form CT-1 X. Otherwise, the IRS may assess an “averaged” FTD penalty. See “Averaged” FTD penalty in section 11 of Pub. 15 for more information about “averaged” FTD penalties. The total tax reported on Form 945-A, line M, must match the corrected total tax (Form CT-1, line 15 (line 19 for tax years 2020–2023), combined with any correction reported on Form CT-1 X, line 21) for the year, less any previous abatements and interest-free tax assessments.

Example—You owe tax. On March 14, 2025, you discovered that you underreported $1,000 of RRTA compensation on your 2024 Form CT-1. File Form CT-1 X and pay the amount you owe by March 2, 2026 (February 28, 2026, is a Saturday), because you discovered the error in 2025, and March 2, 2026, is the due date for that year. If you file Form CT-1 X before March 2, 2026, pay the amount you owe by the time you file.

Form CT-1 X can’t be used to correct