The Self Assessment deadline is on 31 January 2016. This is the deadline for the electronic submission of income tax and capital gains tax returns for the year ended 5 April 2015. Missing the deadline by just a day means a £100 fine.
If you have perfect records and bring them in at once, then we can probably make the deadline with all the technology that we use. However, if anything goes wrong then the deadline is likely to be missed, for which we will not be responsible.
Any tax that is due needs to be paid in full by the 30 day deadline on 1 March 2016, or another penalty of 5% of the tax due will be imposed on top of any commercial interest. You should therefore bear in mind that even if you miss the 31 January deadline, there is still something to play for if you take prompt action.
We call this type of deadline a structured deadline. A £100 fine is likely to be small change to some people, but 5% could hurt a bit more.