31st January 2017 is online tax return deadline.
Hmrc Must Receive self-assessment form and pay the tax you owe.
Following are Important deadline which you must be aware of
Self Assessment Deadline
Register for Self Assessment 5 October 2016
Paper tax returns Midnight 31 October 2016
Online tax returns Midnight 31 January 2017
Pay the tax you owe Midnight 31 January 2017
Paper Tax Return date for tax year 2015/2016 is already gone and now only available option is only tax return by 31st Jan 2017.
If anyone miss this deadline that person will usually pay penalty and interest on outstanding amount owe to HMRC. Someone can appeal against penalty if they have reasonable excuse.
HMRC only accepts serious reasonable excuse such as some died or Tax payer is in hospital or serious life threatening illness.
HMRC allows trustee of a registered pension scheme or non-resident company to submit paper returns by 31st Jan following tax year which is 31st Jan 2017 for tax year 2015/2016.
HMRC might also email or write to you giving you a different deadline.
Partnership returns if you have a company as a partner
If your partnership’s accounting date is between 1 February and 5 April and one of your partners is a limited company, the deadline for: 
- online returns is 12 months from the accounting date
- paper returns is 9 months from the accounting date
Above mentioned information is taken from HMRC website.
