I wish I had done some research on the system of the civil reg indexes before I went to the FRC in Clerkenwell. I heard that it had indexes of England and Wales births, marriages and deaths from 1837 onwards. I went there over 20 years ago hoping the indexes would give the dates of birth at least. I went there and looked for the birth registration of a family member born 1895. I saw the entry, and it was in the Jul, Aug, Sep quarter 1895, Oxford, first name and surname, and vol and page number. I thought "Where is the date of birth?". I asked a lady beside me and she said you have to buy the certificate to get the full details.
My uncle was born in September 1944, and I looked in the huge ledger book under the relevant surname for births registered in July, Aug and September 1944, as I knew his names and mothers maiden name. Could not find his birth, and I thought "Strange".
I tried births registered in October, Nov and Dec 1944 and found it. I thought, well if he was born mid September, why was it registered during Oct, Nov or Dec? Was he slightly younger than thought? Did he even know his actual birthday? I then found out you were given 6 weeks to register a birth, and sometimes it was registered later than that. So if born mid September 1944, his birth was probably registered in October 1944, so therefore would appear in the births registered in October, Nov and Dec 1944, which is usually not the day of the actual event.
So if anyone said they were born in mid Dec 1944 and could not find their birth in the indexes for births registered in October, Nov and Dec 1944 then if their birth was registered in January 1945, it would be under births registered in Jan, Feb, Mar 1945. If they looked further and found their birth in the next quarter, which was first quarter of 1945, and were unaware of the time given to register a birth, they may have thought they were actually slightly younger than believed to be, until they asked at the desk and were told a 6 week period was given to register a birth.