YEARSPERREPEAT
int cycles = (newp->tm_year - oldy) / YEARSPERREPEAT;
int tmy = oldp->tm_year + cycles * YEARSPERREPEAT;
yrem = wantedy % YEARSPERREPEAT - y % YEARSPERREPEAT;
dayoff = ((wantedy / YEARSPERREPEAT - y / YEARSPERREPEAT
+ yrem / YEARSPERREPEAT - (yrem % YEARSPERREPEAT < 0))
wantedy = y + (yrem + 2 * YEARSPERREPEAT) % YEARSPERREPEAT;
#define AVGSECSPERYEAR (SECSPERREPEAT / YEARSPERREPEAT)
enum { years_of_observations = YEARSPERREPEAT + 2 };