The deadline for submitting self assessment tax returns online is 31st January following the end of the tax year:
For example: 2015/2016 tax year
Year end is 5th April 2016
Deadline is 31st January 2017
The deadline for submitting limited company accounts to HMRC is 12 months from your year end. However, the tax needs to be paid 9 months and 1 day from the end of your accounting year.
The deadline for submitting limited company accounts to Companies House is 9 months from your financial year end.
However, this can be different for your first year if you have extended your first financial year so that it is longer than 12 months.