If you have ever wondered why the UK tax year starts on 6 April, then please see John Barnett's post. October represents the halfway point of a UK tax year and the clocks will also go back later this month. It is therefore a good time to remember that a "day" is calculated differently for UK tax and immigration purposes.
"Days spent in the UK" is a crucial concept to the Statutory Residence Test ("SRT") which was introduced by the Finance Act 2013. Broadly speaking, an individual will be treated as having spent a day in the UK for tax purposes under the SRT if the individual is present in the UK at midnight at the end of that day. There are of course exceptions to this rule and the definition of "day" is different in some parts of the SRT (e.g. "work days").
Whilst an individual's "presence" in the UK is key for UK tax purposes, UK immigration law looks at an individual's "absences" from the UK. Section 50(10)(b) of the British Nationality Act 1981 (the "Act") provides that any reference to a day (in the Act) on which a person was absent from the UK is a reference to a whole day. The Home Office guidance, "Indefinite leave to remain: calculating continuous period in UK (published on 4 June 2020)", also confirms that part-day absences (i.e. under 24 hours) are not counted.
In practice, it is common for clients (and even advisers!) to assume that an individual is resident in the UK for tax purposes by virtue of the individual's UK immigration status (e.g. Tier 1 (Investor) visa holder). Although it may be difficult (since the 2018 changes) to meet the "continuous residence" requirement for permanent residence in the UK (i.e. settled status or Indefinite Leave to Remain) whilst remaining non-UK resident for tax purposes, it is possible (albeit not forever!) if the relevant rules are followed.
Apart from counting stamps in a passport and calculating the number of days on an Excel spreadsheet, expert advice should be sought from advisers who can analyse how the UK tax and immigration systems interact.
On this first day of the tax year it's worth recalling why the UK's tax year starts on 6th April.