It is supposed to keep time even when the device is shut off. Check the polarity of the battery, I have seen one where the battery was installed backwards. The positive terminal should be closest to the female five-pin connector, and near the "+" indicator. It is also possible that the RTC module is bad, I have seen that before too. But typically if the Kenbak-1 is keeping the correct time while it is turned on, then the RTS is operating correctly.
So I suspect the battery is either backwards or dead.
Let me know what you find. Sending another RTC module to Brazil would be kind of expensive, but I see they are available on Amazon.com.br
, so if you have to get another one I can reimburse you for it.