def filter_stale_servers(candidates, primary = nil)
        return candidates unless @max_staleness
        max_staleness_ms = @max_staleness * 1000

        if primary
          candidates.select do |server|
            validate_max_staleness_support!(server)
            # last_scan and last_write_date are Time objects. Subtracting them gives a duration in seconds (Float)
            staleness = (server.last_scan - server.last_write_date) - # seconds
                        (primary.last_scan - primary.last_write_date)  + # seconds
                        (server.heartbeat_frequency_seconds * 1000) # ms
            staleness <= max_staleness_ms
          end
        else
          max_write_date = candidates.collect(&:last_write_date).max
          candidates.select do |server|
            validate_max_staleness_support!(server)
            # Same issue here: max_write_date and last_write_date are Time objects
            staleness = max_write_date - server.last_write_date + (server.heartbeat_frequency_seconds * 1000)
            staleness <= max_staleness_ms
          end
        end
      end