Deadlinedated and time limited

The UK tax year runs 6 April to 5 April and every deadline hangs off it

5 Aprilthe tax year ends, and the deadline chain for that year begins

Fahad ZarCrypto tax accountant. MSc Accounting and Finance, BPP University London. Four years reconciling wallet and exchange histories for UK and US filings.

Published 25 August 2026· Reviewed against gov.uk 25 August 2026· 4 min read·Self Assessment and side income

The UK tax year has refused to match the calendar since the eighteenth century, and every deadline that matters is anchored to its odd shape. It starts on 6 April and ends on 5 April the following year. The year that just ended ran 6 April 2025 to 5 April 2026, and its paperwork is what most people are dealing with now. The current year runs to 5 April 2027.

Once you hold that one axis, the rest of the calendar is mechanical, per GOV.UK.

One axis, 6 April to 5 April, then four dates, 5 October, 31 October, 31 January, 31 July. Everything else is a special case.

The deadline chain, for the years in play

The 2024 to 2025 filing deadline has passed. If that year is still unfiled, the penalty clock is already running and the useful move is filing now rather than waiting for the anniversary.

The two dates people actually miss

5 October is the quiet one. It is not a payment date, it is the date HMRC has to be told you need to file at all, and missing it opens a separate failure to notify penalty. First gain, first side income over the thresholds, first year of dividends over the allowance, all of them start here. Registering produces the UTR you cannot file without, and it arrives by post at its own pace.

30 December is the useful one. File online by then and a bill under £3,000 can be collected through your tax code across the following year instead of landing as one January payment.

The deadlines that ignore the tax year

UK residential property gains have their own 60 day clock from completion, which runs whatever the calendar says, the CGT timing guide covers it. VAT, PAYE and Corporation Tax run their own cycles. And loss claims run on a four year limit from the end of the tax year the loss arose in, which is the deadline that quietly expires while people wait to need it.