+128731 It looks like you want to know the time 30 minutes earlier than 5:09
There will be 9 minutes from 5:09 back to 5:00
Then, 21 minutes earlier than this will be the time you want = 4:39
Another way to see this is to look at the time 30 minutes before 5:00 = 4:30
Adding 9 minutes to 5:00 = 5:09 ....so, likewise, adding 9 minutes to 4:30 = 4:39
