How to Edit or Change Your Accounting Period Dates

Need to file for dates that do not match the default dropdown? This guide explains how to enter custom accounting period dates in TinyTax.

When You Might Need Custom Dates

• Your CT603 Notice to Deliver from HMRC shows dates that differ from the dropdown options
• You are filing a final return for a company that ceased trading mid-year
• Your company started trading mid-year after a period of dormancy
• You have changed your Accounting Reference Date (ARD) with Companies House
• HMRC has rejected a return due to an overlapping period and asked you to refile with corrected dates

How to Enter Custom Dates

Custom dates are entered on the filing form, using the Need to file for different dates? link — there is no "CT600 with custom dates" item in the dashboard menu.

1. On your dashboard, find the accounting period and click
   Start Filing
   , then choose CT600 only. (If you have already started this return, open the menu on that row and choose Edit Form instead — it reopens the same form.)
2. On the form, just under the Accounting Period, click the link Need to file for different dates?
3. A box appears — "Why do you need different dates?" — with three choices. Pick the one that fits and click Continue:

- Company closing down — filing a final return with an earlier period end date - HMRC period differs from Companies House — your CT603 Notice to Deliver shows different dates - Other reason — changed year-end, historical filing, etc.

1. The period date fields become editable — enter your dates exactly as shown on your CT603
2. Complete and submit the form as normal

What Dates Should I Enter?

Field	What to enter
Period Start Date	The first day of the accounting period (shown on your CT603)
Period End Date	The last day of the accounting period (shown on your CT603)

If you are unsure which dates to use, check your CT603 letter from HMRC — it specifies the exact period start and end dates that HMRC expects.

Common Questions

I can't find a "CT600 with custom dates" option

There isn't one — that wording is out of date. The option to change your dates lives on the filing form, not in the dashboard menu. Start the return (Start Filing → CT600 only, or Edit Form from the row's menu if you have already started one), then on the form click Need to file for different dates? just under the Accounting Period, and pick the reason that fits. The date fields then become editable.

If the period you want is not showing on your dashboard at all (for example it still says "Too early", or the window has not opened yet), contact TinyTax support and we will help you file it.

The Accounting Period doesn't show my dates — what do I do?

Use the steps above to enter custom dates on the form. The dropdown shows periods based on your Companies House year end, but HMRC may require dates that differ from these — the Need to file for different dates? link lets you type the exact dates in.

Can I edit dates for a Companies House accounts filing?

Custom dates apply to CT600 filings only. Companies House accounts must use your registered Accounting Reference Date (ARD). If you need to change your year end with Companies House, file form AA01 via Companies House WebFiling.

I entered the wrong dates — can I change them?

If you have not submitted yet, simply go back to the form and correct the dates. If you have already submitted, see the section below.

HMRC rejected my return because of an overlapping period — what do I do?

This happens when the period start date overlaps with a previous CT600 already on HMRC's record. The most common cause is a leap year edge case: if your previous return ended on 29 February (a leap year date), the next return must start on 1 March — not 29 February.

To file the corrected return:

1. Go to your dashboard and open the company
2. Find the CT600 row for the relevant period and click Start Filing → CT600 only (or Edit Form if you already started it)
3. On the form, click Need to file for different dates? and choose HMRC period differs from Companies House
4. Enter the corrected dates — ensure the start date is the day after the previous return's end date (e.g. if the previous return ended 29/02/2024, start this one on 01/03/2024)
5. Complete and submit as normal

The HMRC-rejected submission does not need to be cancelled — because HMRC did not accept it, there is nothing to undo. Simply filing with the corrected dates will satisfy the outstanding obligation.

HMRC accepted my CT600 but says the period dates are wrong (or it isn't showing in my HMRC account)

This happens when HMRC's own accounting-period history differs from your Companies House dates — most often after a year-end change, where your HMRC Corporation Tax year-end was different from your Companies House year-end. TinyTax builds your return from your Companies House record, so the start date can differ from the one HMRC expects.

There are two situations, and they are handled very differently:

1. The period HMRC expects has a different END date (for example a shorter or earlier period)

You can file a new CT600 for HMRC's period. Because the end date is different, HMRC does not treat it as a duplicate.

1. Check your CT603 letter for the exact dates HMRC expects
2. Go to your dashboard and open the company
3. Find the CT600 row and click Start Filing → CT600 only (or Edit Form if you already started it)
4. Click Need to file for different dates?, choose HMRC period differs from Companies House, and enter the dates from your CT603
5. Complete and submit as normal

2. Only the START date differs — the END date is the same, and your return is already accepted

Because a return for that period end has already been accepted by HMRC, TinyTax will block a second submission ("You have already filed CT600 for this period… The submission was accepted") — and an in-app amendment keeps the same period dates, so it cannot move the start either. Re-filing is not the answer, and you will not be charged again to sort this out.

The return is genuinely lodged with HMRC — you can confirm this from your acceptance receipt (your IRmark). Only HMRC can realign the accounting period on a return they have already accepted. Contact HMRC's Corporation Tax helpline, quote your UTR and the IRmark from your submission confirmation, and ask them to confirm — and if needed realign — the accounting period the accepted return is matched to. TinyTax support can help you find your IRmark / receipt if you need it.

Your Companies House accounts are a separate filing and are not affected by any of this.

HMRC says I still owe a return for a period that is not on my dashboard at all

This usually means HMRC is holding a short "stub" period that Companies House never had — often only a few days or weeks, sitting between two of your filed years. Support can add any period to your dashboard, with whatever dates HMRC requires. See our guide on a period missing from your dashboard.

I entered the wrong dates and already submitted — can I amend?

See our amending your CT600 guide for the amendment process.

TinyTax is still showing my old accounting period after I changed my ARD

TinyTax syncs your filing periods from Companies House, not directly from HMRC. There are two common scenarios:

You notified HMRC but have not yet filed form AA01 with Companies House Companies House and HMRC hold separate records of your accounting period. Until you file AA01 with Companies House, TinyTax will continue to show the old dates. Use the steps above (Need to file for different dates? on the form) to file for the new period immediately, and submit the AA01 to update Companies House.

You have already filed AA01 with Companies House TinyTax syncs from Companies House automatically — the new period should appear within 24–48 hours of Companies House processing the form. If it has been longer than that, contact support.

In either case, you do not need to wait for the sync — use the Need to file for different dates? link on the form to file straight away.

I changed my ARD and now have two periods to file

When you shorten your accounting period mid-year, you will have two separate CT600 obligations:

1. A standard period up to your old year-end (e.g. 1 May 2024 to 30 April 2025)
2. A shortened period from the old year-end to your new year-end (e.g. 1 May 2025 to 31 October 2025)

File the earlier period first, then use custom dates for the shortened period. HMRC expects returns in chronological order.