Something

#ifndef SECSPERREPEAT
#define SECSPERREPEAT           ((int_fast64_t) YEARSPERREPEAT * (int_fast64_t)AVGSECSPERYEAR)         
#endif /* !defined SECSPERREPEAT */


static int
differ_by_repeat(t1, t0)
const time_t    t1;
const time_t    t0;
{
        if (TYPE_INTEGRAL(time_t) &&
                TYPE_BIT(time_t) - TYPE_SIGNED(time_t) < SECSPERREPEAT_BITS)
                        return 0;
        return t1 - t0 == SECSPERREPEAT;
}