Filing Deadlines

Hi Sir When does the CT return need to be filed for a long period of account, say: 01 Jan 2018 to 31 March 2019? Is it 12 months after the end of the period of account (I.e. submit the return by 31 March 2020) or 12 months after the first accounting period of the two accounting periods (I.e. submit by 31 December 2019)? I’m trying to help a colleague, and I cannot recall. I tried rewatching some of your videos, but couldn’t find it. Thank you