When is the tax year?

In the UK, people get taxed on what they earn from 6 April to the following 5 April.
So for example, the 2023/24 tax year runs from 6 April 2023 to 5 April 2024.
If you are employed and not a high earner (£100k a year+, or £50k a year if you get child benefit), you don't need to do anything beyond that as your employer would have taken off your tax.
If you are self-employed, you will need to complete a tax filing, called a tax return (don't confuse the word return with refund though - most people have to pay!).
The filing needs to be completed by 31 January after the tax year ends (there were some extensions around COVID). So for the year ended 5 April 2023, the filing is due by 31 January 2024. If you have earned more than the personal allowance (which is around £12,500 depending on the year), you will need to pay tax as well. Some people put money aside as they earn.
Normally, if you submit after the deadline you'd be liable to a £100 penalty.  You also need to pay tax due by 31 January (and possibly an amount by 31 July - see What are payments on account?). But so long as you pay within 30 days of the due date, there's no penalty (there will be a small amount of interest). 
We always recommend that people complete a tax return as soon as possible. The quickest and easiest way to do this is with untied.
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