code: func (t *TombstoneGC) nextExpires() time.Time {
  	// The Round(0) call here is to shed the monotonic time so that we
  	// can safely use these as map keys. See #3670 for more details.
  	expires := time.Now().Add(t.ttl).Round(0)
  	remain := expires.UnixNano() % int64(t.granularity)
  	adj := expires.Add(t.granularity - time.Duration(remain))
  	return adj
  }
docs: // nextExpires is used to calculate the next expiration time, based on the
  // granularity that is set. This allows us to bin expirations and avoid a ton
  // of timers.