I was really wondering about that (the twins spanning midnight) too!
FWIW, in the UK, I would have bribed the midwife to put Sept 1st on a birth certificate if a DC was born at 11:59 on Aug 31st. All statistics point to older DC doing better certainly at school if not 'in life' in general. Note I put statistically, so that those who pile on to tell us how brilliantly their Aug 31st 4 year old is doing can step away now- unless they are keen to prove they don't understand what statistics are and mean!
Had I been younger, ie with more 'conception time' to play with, I would have attempted a mid Jan birthday to advantage my DC in both possible education systems, the one they were born into abroad, and here. As it is, they're both May! (though a MC in between would've been Feb).
Oxbridge is disproportionately filled with 1st term birthdays, too, Ice.
Finally, I recall in the Sloan Ranger Handbook or maybe Diary (anyone remember that?!) that they mentioned Breeding Season, for gun dogs, horses- and parents- in order to try and make sure Hugo or Jocasta was amongst the eldest at Harrow or Downe House, I gather
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