pgtime.h documented the PG interpretation of tm_mon right alongside
the POSIX interpretation of tm_year, with no hint that neither
comment was correct throughout our code.
Perhaps someday we ought to switch to using two separate struct
definitions to provide a clearer indication of which semantics are
in use where. But I fear the tedium-versus-safety-gain tradeoff
would not be very good.
Discussion: https://postgr.es/m/CAJ7c6TOMG8zSNEZtCn5SPe+cCk3Lfxb71ZaQwT2F4T7PJ_t=KA@mail.gmail.com
Tom Lane
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