I gave your question some thought, but the answer I give you assumes you need to turn off your Raspberry Pi to conserve batter power. If that is not the case, there is always the software answer: just shut the services down (put it on some init level that shuts everything down, you can even make a specific init level for that and not use init 0 or init 1 or init 6), except for cron, which you would need to set the system into that state and then to reboot it (or if you are skilled enough and have the time and courage - to switch init back to level 2 or 3). That would solve your question, if power saving was not the main issue behind it.
Considering the need to save energy, I think it might be possible with a custom firmware, and maybe an external RTC, to build an energy-saving solution without external circuitry. But I don't think that would be the case, very complicated, time consuming and error-prone.
But the ready answer I have to power saving would be to have an external circuitry of some kind perform the hard reset as M Noit commented in your question, using the P6 connector.
Considering that solution, then the easiest way would be not having to print some circuit or keep some unreliable bredboard connected, you could use the ATmega microcontroller on the Gertboard to perform the hard-reset for you. That reset circuit is described in the D9 sector of the first page of the raspberry pi v.2 schematics. As per that schematics, a simple GND signal on P6-1 should do the trick. I know of no way to determine if the input current on the ATmega output pin is enough to drive that circuitry connected to the D15-RUN pin on the BCM2807 (no datasheet available!) but if not, the Gertboard has buffer circuits and open collectors that will do the trick for sure. Just put the ATmega pin into a high Z or input state when not signaling DOWN to take it out of the circuit. Worst case would be having to use the perforated section of the Gertboard for a relay to just connect the two pins of P6, as the design of the Raspberry Pi intended its use.
The ATmega itself has no RTC, but you can connect one to it, or just calculate the amount of time it will take for the next powerup when powering the Raspberry Pi down, and send it at power-down time to the ATmega.
This answer has some pointing to hardware building and solution. I have run no tests, and I have not implemented anything I just described. So use it at your own risk, knowing that it can brick your ATmega and/or your Raspberry Pi.