Income tax is charged for each tax year separately
The income tax you pay is calculated on your income in the tax year. The tax year runs from 6 April in one year to 5 April in the next. Your tax free Personal Allowance, as well as the tax rates and bands, depend on which tax year you are looking at. Sometimes there are big differences in the way income is taxed between tax years, so it is very important to know the rules for the tax year you are looking at. Don’t assume it will be just the same as last year.
Tax years are normally referred to by the year they start and end in (or even just the year they end in), so the tax year starting on 6 April 2018 and finishing on 5 April 2019 is called the tax year 2018/19.