Corporation Tax Filing Date

Have the rules for filing first period CT returns changed? Usually HMRC send a Return Reminder showing that a return is due 24 months after incorporation (please just assume that the company begain trading on the day they incorporated) and then another a few days later for the rest of the long period. Recently however, the reminders we are getting show the filing date for the first 12 months as being the last day of the 12th month and not the date I would expect.

Does anybody know if the rules have changed or if the HMRC letter is wrong?