There is some cumulative error (maybe one second per hour) after sleeping in increments of 8 seconds (the longest interval possible with the watchdog timer) because of the time lost starting up and shutting down, so this approach is not 100% accurate.
Running direct from 3xAA batteries (=4.5V) means that you can avoid a voltage regulator from 9V too.
http://donalmorrissey.blogspot.de/2010/04/sleeping-arduino-p...
There is some cumulative error (maybe one second per hour) after sleeping in increments of 8 seconds (the longest interval possible with the watchdog timer) because of the time lost starting up and shutting down, so this approach is not 100% accurate.
Running direct from 3xAA batteries (=4.5V) means that you can avoid a voltage regulator from 9V too.