YEARSPERREPEAT
years = (seconds / SECSPERREPEAT + 1) * YEARSPERREPEAT;
yearlim = EPOCH_YEAR + YEARSPERREPEAT;
yearlim = year + YEARSPERREPEAT + 1;
#ifndef YEARSPERREPEAT
#define SECSPERREPEAT ((int_fast64_t) YEARSPERREPEAT * (int_fast64_t) AVGSECSPERYEAR)
enum { years_of_observations = YEARSPERREPEAT + 2 };