SECSPERLYEAR
seconds = isleap(myy) ? SECSPERLYEAR : SECSPERNYEAR;
seconds = isleap(myy - 1) ? SECSPERLYEAR : SECSPERNYEAR;
+ SECSPERLYEAR * (intmax_t) (100 - 3))
enum { SECSPER400YEARS_FITS = SECSPERLYEAR <= INTMAX_MAX / 400 };