Well, an alternative has been suggested, that you use the "wake" interrupt to respond to the motion sensor, given that you intend no other activity to be required in the absence of motion detected.
In which case, depending on how much motion activity you anticipate in the "off" period, you have two choices.
The first is to wake only on motion sense and check the clock to see if you "care" about motion at that time in which case you do not need to wake on the alarm.
The second is to respond to the alarm waking interrupt in order to enable motion interrupts and if on any given motion interrupt, it turns out you are past the time that it matters, you can then disable the motion interrupts again until the next alarm. This would absolutely minimise the amount of time you are "awake".