[Ethereum Alarm Clock](http://www.ethereum-alarm-clock.com/).

This service claims that it itself is implemented as a smart contract, but how is that possible? I thought the whole reason it existed was because [A contract can't wake up itself](https://ethereum.stackexchange.com/questions/304/why-cant-a-contract-wake-up-and-run-itself), but isn't that what this alarm clock has to do to run/call other contracts?