You have until 31 January but don’t leave it until the last minute
Yes, you can file without penalty until 28 February this year, but leaving it until the last minute is fraught with problems. If you are not registered with HMRC to complete your tax return online, you will need an activation code, which the website says will take up to 10 days to arrive. Even if that is in hand, you need to give yourself time to track down the right facts and figures, and to check that everything is correct.