the RTC chip used for real time backup doesn't have this alarm output hardware implemented,
So, if you want this function, you'll have to program it by yourself.
For example you could ask the program to read the actual time every minut or second (depending what accuracy you need) and compare to an alarm time you previously decide.
I think you'll be able to find some existing alarm clock using DS1307 and arduino
there is one here : http://hacks.ayars.org/2010/01/alarm-cl ... rkill.html
hope it helps,