SimpleReturns

What are boxes 30 and 35 on the CT600? (your accounting period)

Reviewed by Lee Jones, Founder · Updated 16 July 2026
The short answer

Boxes 30 and 35 are the first two date boxes on your Company Tax Return. Box 30 is the day the return's period starts and box 35 is the day it ends. For most companies they simply match the year in your annual accounts, but there is one famous catch: a tax return cannot cover more than 12 months, so a long first year needs two returns.

Official source. This guide is a plain-English summary of official GOV.UK guidance, not advice. The authoritative source is The Company Tax Return guide on gov.uk. Always rely on that over our summary.

What goes in them?

HMRC's own guide puts it in one line: enter the beginning date of the period in box 30 and the end date in box 35. That period is called your "accounting period" for Corporation Tax, which is just the stretch of time the return covers.

For almost every company, it is the same year your annual accounts cover. If your accounts run from 1 June 2025 to 31 May 2026, then box 30 is 1 June 2025 and box 35 is 31 May 2026. No sums, no lookups, just the two dates off the front of your accounts.

They matter more than they look. Every other number on the return, from your turnover to the tax itself, is measured between these two dates. If you filed last year, this year's box 30 should normally be the day straight after last year's box 35, with no gap and no overlap.

What usually goes wrong?

The classic trap is the first year. It catches people out because of how the dates are set up, not because anyone did anything wrong.

When you set up a company, Companies House gives it a year end on the last day of the month you registered in, one year later. So a company set up on 11 May 2024 gets a year end of 31 May 2025, and its first accounts cover a little over 12 months. That is completely normal. The snag is on the tax side: a Company Tax Return cannot cover more than 12 months.

So a long first year means one set of accounts but two tax returns:

  1. The first return covers the first 12 months, so its box 30 is 11 May 2024 and its box 35 is 10 May 2025.
  2. The second return covers the leftover stub, so its box 30 is 11 May 2025 and its box 35 is 31 May 2025.

Each return gets its own boxes 30 and 35, its own share of your profits, and its own payment deadline. The trap is that nobody sends you a warning about the second one. Plenty of directors file the first return, breathe out, and never realise a second one is due. A missed return costs £200 from the very first day it is late, even if the company made no profit in those few weeks.

Do I have to work this out myself?

No. Any decent filing software fills these boxes for you, and it should also spot the long first year on its own.

SimpleReturns reads your dates from your Companies House record, fills boxes 30 and 35 for you, and if your first period runs longer than 12 months it prepares both returns automatically, splitting the figures between them so nothing is filed short or filed twice. You review every date and every figure before anything is sent. It is free to start, no card needed, and one flat £99 covers the filing, all the tax returns to HMRC and the accounts to Companies House.

If your company is part of a group, or has been chopping and changing its year end, an accountant is genuinely the right call, and we would rather tell you that plainly than pretend otherwise.

Common questions

My first accounts cover more than 12 months. Do I really have to file two tax returns?

Yes. The accounts can be longer than 12 months but a tax return cannot, so the period gets split into a 12-month return and a short second return that mops up the remaining days. One set of accounts covers both.

Do boxes 30 and 35 have to match my accounts exactly?

Normally yes, and for most companies they are identical to the accounts year. The exception is a period longer than 12 months, where the accounts stay whole but the tax side is split across two returns.

What happens if I only file one return for a long first year?

The second return is still due, and once its deadline passes it is late. Late means a £200 penalty from day one, so it is worth checking now whether your first accounts run past 12 months.

Want the dates filled in for you?

SimpleReturns works out your accounting period from your Companies House record, fills boxes 30 and 35, and handles the two-return first year automatically. Free to start, no card needed, and £99 flat covers all the filings.

Start your return

And if your company's situation is genuinely unusual, say a group or a string of year-end changes, we will say so rather than guess.

General guidance, not advice. This guide explains how the rules generally work for small UK limited companies. It isn't tax advice for your specific situation, if you're unsure, check with us or an accountant.