Because the alarm clock is half a minute slower than the standard time every hour.

So the standard time is 60 minutes and the alarm clock is 59.5 minutes.

Set the standard time to x minutes after 8 pm.

( 12-8+5)*60+55=595

60:59.5=x:595

x=600

600 minutes = 10 hour

10-( 12-8)=6.

So this alarm clock will go off at 6: 00 standard time.