SECSPERNYEAR
seconds = isleap(myy) ? SECSPERLYEAR : SECSPERNYEAR;
seconds = isleap(myy - 1) ? SECSPERLYEAR : SECSPERNYEAR;
#define SECSPERLYEAR (SECSPERNYEAR + SECSPERDAY)
#define SECSPER400YEARS (SECSPERNYEAR * (intmax_t) (300 + 3) \