@hikari musl approach: simple, all fairly intelligible numbers; lots of modulo; some explicit branching for edges: https://github.com/bminor/musl/blob/f5f55d6589940fd2c2188d76686efe3a530e64e0/src/time/__secs_to_tm.c#L35
the libcxx approach: just do a bunch of divides by constants lol https://github.com/llvm/llvm-project/blob/51eeea67c67cb3e622730ee1fa8c9b939268429b/libcxx/include/__chrono/year_month_day.h#L75