I already tried this one too, but couldn't fix it with TIMSK
I am that desperate, that I am thinking about using a real time clock which works even if the arduino is in powerdown mode (like that one from China: http://www.ebay.de/itm/DS1302-Real-Time-Clock-Module-mit-CR2032-3V-Batterie-fur-Arduino-NEU-/191136140796?pt=Wissenschaftliche_Geräte&hash=item2c809a0dfc) but i don't know if this is the best alternative...