In New Zealand, every kid can begin school on the child's fifth birthday. There is then no particular birthday cut-off for schools, other than that kids might be stuck in Year 0 a little bit longer.
In much of the rest of the world, school starts on a particular date: if you turned 5 before some cut-off date, you're in; if not, it's another year of daycare, but you'll be one of the older kids in your class. This has consequences for timing of births.
New Zealand may be considering flipping to the "everyone starts on the same date" regime. And so you have the potential for a rather nice difference-in-difference study: none of the kids in the current cohort would have been born with parents anticipating school-start timing effects around a set start date, but if the regime changes, they will - in a few years. In the transition, there will be kids in one year who started on their birthday and others just starting on the set date.
If it happens, I wonder whether anybody will pick this up as a neat identification strategy.