Why is the UK tax year 5 April?
Answered 17 March 2026
The retrieved sources — both the initial set and the additional searches — do not contain any document explaining the historical reason why the UK tax year ends on 5 April (i.e. the story of the 1752 calendar reform, Lady Day, and the 11-day adjustment). This is a matter of historical/constitutional background rather than current tax law or HMRC guidance, and it does not appear in the available knowledge base materials.
I could not find an authoritative source for the historical origin of the 5 April tax year end in the available materials. The retrieved documents confirm that the UK tax year runs from 6 April to 5 April, but none of them explain why that date was chosen.
For a definitive explanation, you may wish to consult:
- The House of Commons Library (which has published briefing papers on this topic), or
- HMRC's own historical notes, which explain that the date derives from the adoption of the Gregorian calendar in Britain in 1752. Under the old Julian calendar, the tax/fiscal year began on Lady Day (25 March). When 11 days were "lost" in September 1752 to align with the Gregorian calendar, the Treasury adjusted the start of the fiscal year to 5 April (25 March + 11 days) to avoid losing tax revenue for those days — and it has remained there ever since.
This historical background is not covered by the HMRC guidance or legislation in the available knowledge base, so I am unable to cite a source for it here.
Citation sources
You’ll need your: National Insurance Number employer PAYE reference number — if you have it parts 2 and 3 of your p45 — if not tell us why (for example, you may be retired, or a UK Crown servant employed abroad) self-employment profits made in this tax year — if there are any Your P45 shows how much tax you’ve paid on your salary so far in the tax year (6 April to 5 April). You’ll also need to confirm if you’ve had any: pensions from a former employer public service pensions forces pensions pers