The deadline for filing the 2015/16 self-assessment online is 31 January 2017. It is important that this deadline is not missed because if you miss it, even by one day, you will have to pay a late filing penalty of £100.
Don't miss the tax return deadline. Personal tax returns for the 2014/15 tax year (which ended on 5 April 2015) must be filed online by 31 January 2016 at the latest. Any remaining tax due for the 2014/15 tax year must also be paid by this date.