CTM93030 | CTSA: the filing obligation: filing date: definition
From HM Revenue & Customs · Company Taxation Manual
The filing date for a company tax return is the last day of whichever of the following periods is the last to end:
twelve months from the end of the period for which the return is made - see example 1 in ,
three months from the date when the notice requiring the return is served - see and example 2 in for more detail,
if the company’s relevant period of account is not longer than 18 months:
twelve months from the end of that period - see example 3 in ,
if the company’s relevant period of account is longer than 18 months:
30 months from the beginning of the period of account - see example 4 in .
Paragraph 14 (2) defines ‘relevant period of account’. It means in relation to a return for an accounting period, the period of account of the company in which the last day of that accounting period falls. There are examples at .