Wrong HMRC Corporation Tax period dates: how to correct them

Short version. If your HMRC Corporation Tax period dates look wrong, compare them with your notice to file, Companies House accounts dates and activity records. Different dates are not, by themselves, an error. If HMRC's dates are incorrect, contact HMRC to resolve the discrepancy. Changing a date in filing software does not itself correct a notice or change your company year end.

1. Compare the records before changing anything

Gather your company number, Unique Taxpayer Reference (UTR), accounts dates, notice to file (often called CT603), and records of when activity started or stopped. Check whether HMRC is expecting a first return, a short second return, or a period after trading stopped.

A common first-year mismatch is that accounts start on incorporation while trading starts later. Another is a first accounts period longer than one year, requiring two CT600 periods if the company was within Corporation Tax throughout. GOV.UK's first-year guide explains the distinction. Our different-dates guide gives worked examples.

2. Correct the HMRC position

HMRC tells companies to check their accounting period online and report incorrect dates. Follow its contact link or the instructions on your notice. Explain both the dates shown and the dates you believe apply, with the reason for the difference.

Use this as a checklist for the conversation:

• Which tax periods does HMRC currently expect returns for?
• What event explains the proposed start or end, and on what date did it happen?
• Does the existing notice need correcting or withdrawing?
• Is another return needed for any remaining period?
• Which filing deadline and payment reference apply?

Keep the response with your filing records. If the deadline is close or a notice remains unresolved, speak to HMRC promptly. Do not assume that asking for a correction cancels the filing obligation or gives extra time.

3. If you actually want to change the company year end

Changing the period covered by the annual accounts is a Companies House accounting reference date change. Apply through the Companies House process, then address the resulting tax dates. Companies House places restrictions on extensions and does not allow a year-end change when accounts are overdue. A CT600 date edit is not an application to Companies House.

For lengthened accounts, HMRC says to contact it about the tax dates before the original return filing date. For shortened accounts filed using accounting software, its guidance says to enter the new accounting-period dates before filing. If the account or notice still conflicts with the facts, resolve that with HMRC. See the official instructions and our year-end change guide.

4. Before you file

Check the actual return dates, the full accounts attachment and whether more than one return is required. In SimpleCompanyTax, use the separate-date option for a supported HMRC-only filing once you have established the correct period. You keep the annual accounts dates and enter the tax pair separately.

Use the guide to setting different CT600 and Companies House dates if you already know the correct dates. If you already filed using incorrect dates, seek guidance on correcting that filing. Reopening an editable draft does not itself amend an accepted return.

This is general guidance. HMRC can clarify its records and notices. An accountant can help establish which tax periods your activity creates.