Arduino Enerlib library question

I'm looking put the arduino in a low energy state to conserve battery power and i came across the Enerlib in the Arduino Playground page. I'm looking to shut down the arudino to the lowest power mode for 15minutes and than every 15 minutes wake itself up and do something then go back into a sleep/low energy mode. I can't use a hardware inturrent so I think I need to use a watch dog timer. I know the watch dogs are not super accurate but that is ok if it's 15min +/- 5 minutes it's not going to hurt anything. Any ideas on how to get this library to do what I want or any example code using it?

Thanks in advance.