Re: 1970s?
Technically AD years are ordinal and not cardinal numbers - i.e. AD 1 is "the first year after the Lord became present on earth as man" so there is no AD 0.
Also, if he was born on Dec 25 then he clearly was present on earth since conception 9 months earlier on March 25 ... so that marks the start of the year - hence lots of traditional rents etc were paid on "Lady Day".
When the UK switched from the Julian to Gregorian calendar a 12 days were "lost" and one of the contentious issues was whether the government were requireing taxes to be paid "early" so they decided to moved the date when taxes were due by 12 days from March 25 ... and that is why the UK tax year starts on April 6th.
Biting the hand that feeds IT
