CT600 Boxes 30-35: Accounting Period Dates Explained

Boxes 30-35 on the CT600 capture your accounting period dates. Getting these right is crucial - incorrect dates cause submission rejections and can affect your tax calculation.

What Goes in Boxes 30-35?

Box	Description	Format
30	Start of accounting period	DD/MM/YYYY
35	End of accounting period	DD/MM/YYYY
31	Start of return period	DD/MM/YYYY
32	End of return period	DD/MM/YYYY
33	Period covered by accounts start	DD/MM/YYYY
34	Period covered by accounts end	DD/MM/YYYY

For most companies, boxes 30-35 all share the same start and end dates.

Understanding Period vs Return vs Accounts

Accounting Period (Boxes 30 & 35)

The period for which Corporation Tax is calculated:

• Maximum 12 months
• Determines tax rates and thresholds
• Must follow directly from previous period

Return Period (Boxes 31 & 32)

The period covered by this specific CT600:

• Usually same as accounting period
• Different if split period or amended return

Accounts Period (Boxes 33 & 34)

The period covered by your statutory accounts:

• Can be up to 18 months
• May differ from CT600 period for extended periods

Standard Scenario: 12-Month Period

For a typical 12-month accounting period:

Example: Year to 31 March 2024

Box	Value
30	01/04/2023
31	01/04/2023
33	01/04/2023
34	31/03/2024
35	31/03/2024
32	31/03/2024

All dates align for a standard period.

Extended Period (Over 12 Months)

For accounting periods over 12 months, you need two CT600 returns.

Example: 18-month period 1 Oct 2022 to 31 March 2024

First CT600 (12 months):

Box	Value
30	01/10/2022
35	30/09/2023
33	01/10/2022
34	31/03/2024

Second CT600 (6 months):

Box	Value
30	01/10/2023
35	31/03/2024
33	01/10/2022
34	31/03/2024

Note: Accounts period (33-34) covers the full 18 months in both returns.

Short Accounting Periods

Periods can be shorter than 12 months:

• First period after incorporation
• After changing accounting reference date
• Before striking off

Example: 6-month first period

Company incorporated 1 October 2023, first year end 31 March 2024.

Box	Value
30	01/10/2023
35	31/03/2024

Tax thresholds are pro-rated for short periods.

How to Determine Your Dates

Step 1: Find Your Accounting Reference Date

Your accounting reference date (ARD) determines your period end:

1. Check Companies House for your company
2. Look for "Accounting reference date" or "Next accounts due"
3. This is normally your period end date

Step 2: Determine Period Start

Your period starts:

• First period: Date of incorporation
• Subsequent periods: Day after previous period ended

Step 3: Check for Extended Periods

If start to end is over 12 months:

• Split into two CT600 returns
• First: 12 months maximum
• Second: Remainder

Common Date Mistakes

1. One Day Off

Wrong:

• Period start: 31/03/2023
• Period end: 31/03/2024

Correct:

• Period start: 01/04/2023
• Period end: 31/03/2024

The start should be the day after the previous period ended.

2. Overlapping Periods

Each day must be covered by exactly one period - no overlaps or gaps.

Wrong:

• Previous period: Ended 31/03/2023
• Current period: Started 31/03/2023 (overlap!)

Correct:

• Previous period: Ended 31/03/2023
• Current period: Started 01/04/2023

3. Wrong Year

Double-check the year on all dates. A common error is typing 2023 when you mean 2024.

4. Extended Period as Single Return

Periods over 12 months must be split into two CT600s. You cannot file a single return for more than 12 months.

Tax Implications of Period Length

Period length affects:

Tax Rate Thresholds

Thresholds are pro-rated for short periods:

Period Length	Small Profits Limit	Main Rate Limit
12 months	£50,000	£250,000
6 months	£25,000	£125,000
9 months	£37,500	£187,500

Payment Deadline

Corporation Tax is due 9 months and 1 day after period end.

12-month period to 31/03/2024:

• Tax due: 01/01/2025

6-month period to 31/03/2024:

• Tax due: 01/01/2025 (still 9 months + 1 day from period end)

When Using TinyTax

TinyTax asks for your period dates and handles the complexity:

1. Enter your period start and end dates
2. TinyTax validates the dates
3. If over 12 months, automatically splits into two returns
4. Boxes 30-35 are populated correctly

The system warns you about:

• Dates that don't follow your previous period
• Periods that need splitting
• Potential date format issues

Related Articles

• CT600 Boxes Explained
• Extended Accounting Periods
• Changing Your Accounting Period
• First CT600 Return Guide

Need Help?

TinyTax validates your accounting period dates and ensures they align correctly. For extended periods, the software automatically handles the split and populates all date boxes correctly.

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