How to Edit or Change Your Accounting Period Dates | TinyTax Support

How to Edit or Change Your Accounting Period Dates

How to enter custom accounting period dates when your CT603 shows different dates from the dropdown.

Need to file for dates that don't 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're filing a final return for a company that ceased trading mid-year
  • Your company started trading mid-year after a period of dormancy
  • You've 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

Method 1: Dashboard "CT600 with Custom Dates" Option

If your company is already on your TinyTax dashboard:

  1. Find the accounting period on your dashboard
  2. Click
    to open the filing dropdown
  3. At the bottom of the menu, select CT600 with custom dates
  4. A confirmation message will explain that this files CT600 only — click Continue
  5. On the form, the period date fields will be editable — enter your dates as shown on your CT603
ℹ This option files CT600 only. If you also need to file Companies House accounts, file those separately using your standard year-end dates.

Method 2: "Set Final Return Dates" (Closing Company)

If your company is closing and you need a shortened final period:

  1. On the filing form, select a period from the Accounting Period dropdown that hasn't ended yet
  2. A message will appear: "This period hasn't ended yet"
  3. Click
  4. The dropdown will be replaced by editable date fields — enter your final period dates
ℹ Despite the "final return" wording, this also works for entering any custom dates that don't match the dropdown — for example, when HMRC has assigned different period dates.

Method 3: Direct URL Parameter

You can also access editable date fields by adding `?edit_dates=true` to the form URL:

`https://tinytax.co.uk/submission-form.php?edit_dates=true`

This skips the period dropdown entirely and shows editable date fields straight away.

⚠ When editing dates, always use the dates shown on your HMRC CT603 Notice to Deliver. Filing for incorrect dates may cause HMRC to reject your return.

What Dates Should I Enter?

FieldWhat to enter
Period Start DateThe first day of the accounting period (shown on your CT603)
Period End DateThe last day of the accounting period (shown on your CT603)
If you're 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 see the 'CT600 with custom dates' option on the dashboard"

If your next accounting period hasn't started yet (the filing window hasn't opened), the custom dates option may not be visible on the dashboard. Use the direct link instead:

File CT600 with custom dates

This is common when closing a company mid-year — see our filing final returns guide for full details.

"The dropdown doesn't show my dates — what do I do?"

Use any of the three methods above to enter custom dates. The dropdown shows periods based on your Companies House year end, but HMRC may require dates that differ from these.

"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 haven't submitted yet, simply go back to the form and correct the dates. If you've 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
  3. Click
    and select CT600 with custom dates
  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.

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

See our amending your CT600 guide for the amendment process.

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