Your online tax return must reach HMRC by midnight on 31 January.
So for the 2012-13 tax year, the deadline for online returns is midnight on 31 January 2014. There are very few exceptions. As an example, the deadline may be later if HMRC sends you the letter, telling you to complete a tax return, after 31 October. In this case the letter will tell you the deadline - it is usually 3 months from the date of the letter.
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