Self-assessment tax has two deadlines in the year; 31st January and 31st July. The total amount of tax for the last tax year has to be paid by 31st January (so for example the tax year ending 5th April 2016 has to be paid by 31st January 2017). Some taxpayers also have to make payments on accounts for the latest tax year as well. These are paid in two slices; one on 31st January and one on 31st July. So if you have payments on account to make for the year to 5th April 2017 these are due on 31st January 2017 and 31st July 2017. These amounts already paid are then deducted from the balance of tax to pay on the following 31st January.