Wrong HMRC Corporation Tax period dates: how to correct them

Last updated: 15 October 2026

Short version. Check and correct HMRC's Corporation Tax dates before you file, then file new returns for the dates HMRC confirms. A date typed into filing software does not change HMRC's record, and an amendment cannot move the dates of a return HMRC already holds. 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.

Go to the steps and entry details

On this page

HMRC dates firstThen file or amendFirst or later periodsWhere you see it: Company Profile & Period › Accounting period and the Corporation Tax dates

Before entering figures, the filing setup questions can set the existing destination and CT600 route for an ordinary limited company. They keep the full accounts dates beside the CT600 dates. You can also continue with the controls below without opening the questions.

What this guide covers

Check and correct HMRC's Corporation Tax dates before you file, then file new returns for the dates HMRC confirms. A date typed into filing software does not change HMRC's record, and an amendment cannot move the dates of a return HMRC already holds.

HMRC decides which Corporation Tax periods it expects returns for. Your CT603 notice and HMRC business tax account show them.
You correct HMRC's record by contacting HMRC. Nothing in a filing wizard does that.
Once HMRC confirms the dates, you file new returns for them. If they differ from your accounts, you tick the box My HMRC Corporation Tax period is different from my statutory accounts period. How to set different CT600 dates
An amendment corrects a return HMRC already holds, for exactly the same dates. It cannot move those dates. If a filed return has the wrong dates

Is this the right guide for you?

Use it if all of these are true

Your CT603 notice or HMRC business tax account shows Corporation Tax dates you believe are wrong, or a return HMRC has accepted was filed for the wrong dates.
You have the records that show which dates are right: the incorporation date, the accounts dates and when activity started or stopped.

Do not use it if

HMRC’s dates are right but differ from your accounts, for example a dormant company that started trading.Use instead: Dormant company started trading
Your first accounts run for more than 12 months and HMRC expects two returns.Use instead: Standard return
HMRC’s two periods are right but the first ends on a different day from the default split.Use instead: Match your two CT600 dates to HMRC
You changed your company year end at Companies House.Use instead: Changing your company year end

Common mistake: typing your own dates instead of correcting HMRC's record

The Corporation Tax date fields accept the dates you type. They do not change what HMRC expects. A return filed for dates HMRC does not expect leaves HMRC's own record unchanged, and HMRC can still treat a return it expects as outstanding.

Own dates typed in, HMRC not told

HMRC’s record: Unchanged
Return filed for: Dates HMRC does not expect
Result: A return HMRC expects can still be outstanding

HMRC record corrected first

HMRC’s record: Shows the corrected dates
Return filed for: The dates HMRC confirmed
Result: The return matches what HMRC expects

More help: GOV.UK: check your Corporation Tax accounting period · HMRC CTM93020: notice periods and return periods

1. Compare the records before changing anything

Wrong HMRC record: use the HMRC correction steps below.
Established dates, draft not filed: follow the wizard date-entry steps.
Accepted return with wrong dates: resolve the correction route before starting a replacement. Editing a draft alone does not amend the accepted filing. What an amendment can change

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.

Worked example

Company: Holly Example Ltd
First accounts: 1 February 2025 to 28 February 2026, from incorporation
What happened: No income or other activity until trading started on 3 March 2025
HMRC’s record: Corporation Tax period starting 1 February 2025, so HMRC expects two returns: 1 February 2025 to 31 January 2026, and 1 to 28 February 2026
What the company believes: One return from 3 March 2025 to 28 February 2026
Filed so far: Nothing. No CT600 has been sent for these dates

The company and all figures on this page are fictional.

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.

More help: HMRC: Corporation Tax general enquiries · HMRC: active and dormant for Corporation Tax

3. File with the dates HMRC confirms

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 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.

Start a new filing
Dashboard, company card
Click New filing. If you see Resume filing, click the small arrow beside it and choose Start new filing →
Confirm the company
Tick the confirmation box, then click Continue with this company →
Company Profile & Period, if HMRC corrects its record
Accounting period
1 February 2025 to 28 February 2026 (first period)Keep the full accounts period.
Special tax regimes
No
What are you filing?
HMRC CT600
Filing purpose
Standard return
My HMRC Corporation Tax period is different from my statutory accounts period.
Tick itA note under it starts Not needed for a first accounting period longer than 12 months. That note is about HMRC expecting two returns. Once HMRC has confirmed one return from 3 March 2025, tick the box.
Corporation Tax period start
3 March 2025The start date HMRC confirmed.
Corporation Tax period end
28 February 2026
Check the preview
We will prepare one Corporation Tax return: 3 March 2025 – 28 February 2026
These dates match my CT603 notice to deliver a return, or my HMRC business tax account.
Tick it once HMRC’s record shows these dates
Between the start of the accounts period and the Corporation Tax start date above, the company had no income, no chargeable gains and no other activity that brought it within the charge to Corporation Tax. Any costs in that gap are qualifying pre-trading expenditure or are included in the disallowable expenses figure.
Tick it if it is true for 1 February to 2 March 2025
Trading status
ActiveThe form starts on Dormant, so make sure you change it.
How do you want to enter your figures?
Enter my figures once
What the company does
Trading
Click Next →. On Financial Data, enter the figures from the full approved accounts. How the figures are treated
Company Profile & Period, if HMRC confirms its own dates
Accounting period
1 February 2025 to 28 February 2026
Filing purpose
Standard returnA note says two returns will be prepared. It is headed Extended accounting period.
My HMRC Corporation Tax period is different from my statutory accounts period.
Leave untickedHMRC’s two periods follow from the accounts dates.
Change CT600 split date
Leave as shown if the dates match HMRCIf HMRC's first period ends on a different day, set First CT600 ends to that day. How to match both CT600 periods
Trading status
Active
Continue as in the Standard return guide.
Review & file
Click Preview your documents (free) and check that each CT600 shows the dates HMRC confirmed, and that the accounts keep 1 February 2025 to 28 February 2026.
More help: Checking draft document dates

Sample outcome

One CT600If HMRC corrects its record: 3 March 2025 to 28 February 2026, full accounts attached with box 85

Two CT600sIf HMRC confirms its own dates: 1 February 2025 to 31 January 2026, and 1 to 28 February 2026

Accounts unchangedBoth routes keep the accounts at 1 February 2025 to 28 February 2026

4. If a return already filed has the wrong dates

An amendment corrects the figures and details of a return HMRC already holds, for exactly the same dates. It cannot move a return to different dates. This applies to a return filed with us and to one filed elsewhere.

Filed with us: Open the accepted filing, click Amend this filing →, then Proceed to amendment. The amendment keeps the original accounts dates, Corporation Tax dates and first CT600 end, and you cannot edit them. Amending a CT600 with different dates
Filed elsewhere: Start from Amend filing made elsewhere. The dates you enter must match the return HMRC already has, to the day. If they do not, HMRC cannot find the original and rejects the amendment with error 1611.
A new return for the same dates: HMRC rejects a second new return for dates it already holds a return for as a duplicate, error 3304.

The company and all figures on this page are fictional.

So the route starts with HMRC. Tell HMRC which return was filed for the wrong dates and which dates are right. HMRC decides how it treats the return already filed and which returns it still needs. Then use the route that matches what HMRC tells you:

For dates HMRC holds no return for, file a new return with Standard return. If those dates differ from your accounts, also tick this box: My HMRC Corporation Tax period is different from my statutory accounts period.
For a return HMRC holds with the right dates but wrong figures, file an amendment for exactly those dates. Amendment or new filing

Reopening an editable draft does not itself amend an accepted return.

More help: HMRC CTM93300: amending a company tax return · GOV.UK: making changes to a Company Tax Return

When HMRC's dates are correct but differ from the default split

A long accounts period usually splits after the first 12 months, but an earlier event can end the first Corporation Tax period sooner. If HMRC's two periods are correct, keep the full accounts dates. If the calculated first end differs, open Change CT600 split date and edit First CT600 ends to match it. Use the separate Corporation Tax dates if the full tax span differs too. The second return starts the next day and neither may exceed 12 months. See how to match both CT600 periods and the events that end a tax period.

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

If HMRC's Corporation Tax dates differ from the Companies House year end in a later year, read which differences are expected and when to call HMRC. A first accounts period longer than 12 months correctly needs two returns, so the first return's end does not match the 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.

Not the right guide?

Open the guide that matches your situation.

Set different CT600 datesHMRC’s dates are right and differ from your accounts.Dormant company started tradingOne CT600 from the trading start.Changing your company year endA new accounts period and its CT600s.How to amend a CT600Correct the figures in a return HMRC holds.

Looking for a different field or step? See all filing wizard guides →

Official sources

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

Whether this route matches your filings is for you to decide, yourself or with your accountant.

Example companies and figures are fictional. Field names match the filing wizard as of 12 October 2026.

For a long accounts period where only the second CT600 has been filed, file the first CT600 as new and amend the second. The first return is new and the second is amended. This purpose is HMRC only.