code: def before(self, idx):
          
          if not isinstance(idx, datetime):
              raise TypeError("'%s' is not %s" % (idx, datetime))
          day = min(idx.date(), self._hi_limit - DAY)
          while day >= self._lo_limit:
              if day < self._rd_cache.lo or day >= self._rd_cache.hi:
                  self._load(self._rd_cache, day)
              self._rd_cache.set_ptr(idx)
              if self._rd_cache.ptr > 0:
                  return self._rd_cache.data[self._rd_cache.ptr - 1]['idx']
              day = self._rd_cache.lo - DAY
          return None
docs: Return datetime of newest existing data record whose
          datetime is < idx.
  
          Might not even be in the same year! If no such record exists,
          return None.