Those who have not completed the form need to start as soon as possible as it can be a lengthy process.
Make sure you read through the 'How to fill in your Tax Return' booklet and work out if you need to fill in any extra pages.
While there is no reason you should miss the deadline if you act now, if you do not get the form completed in time you will have to be prepared to compensate HMRC.
If you're one day late you'll need to pay a £100 fine and if you leave it longer you can begin to accumulate extra fines of £10 a day.
Check out our guide to completing a self-assessment tax return for a more in depth discussion on how to fill out the form.