« Reply #8 on: Monday 24 July 17 12:39 BST (UK) »
I think it may have been 42 days for quite a while.
My grandfather was born 24th December 1873 and was registered 18 days later - 11th January 1874 so everyone else with him on their tree has 1874 as his birth year. 
I have an ancestor born 24 Nov 1879 and his birth registered in early Jan 1880 so he appears in the Jan, Feb, Mar 1880 quarter, and yes, cousins who also share him as an ancestor have 1880 as his DOB, or even March 1880.
As said the BMD registers are indexed quarterly by the date of the registration, not the event itself. So say for instance someone was born 28th November 1923 but their birth registered on the 2nd January 1924 it will appear in the Jan, Feb Mar quarter of 1924 births.
Researching:
LONDON, Coombs, Roberts, Auber, Helsdon, Fradine, Morin, Goodacre
DORSET Coombs, Munday
NORFOLK Helsdon, Riches, Harbord, Budery
KENT Roberts, Goodacre
SUSSEX Walder, Boniface, Dinnage, Standen, Lee, Botten, Wickham, Jupp
SUFFOLK Titshall, Frost, Fairweather, Mayhew, Archer, Eade, Scarfe
DURHAM Stewart, Musgrave, Wilson, Forster
SCOTLAND Stewart in Selkirk
USA Musgrave, Saix
ESSEX Cornwell, Stock, Quilter, Lawrence, Whale, Clift
OXON Edgington, Smith, Inkpen, Snell, Batten, Brain