Here is your problem:
timerSet.set(20, 8, 2012);
This sets the future date, but not the future time of day (instead, it sets boolean allDay to true), so by default it is 00:00.
From the android API :
set(int second, int minute, int hour, int monthDay, int month, int year)
So try
timerSet.set(0,0,15,20,8,2012)
during 15:00, August 20, 2012
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