Seems any numbers that add up to 20 dont work out.
11:09
1 + 1 + 0 + 9 = 11
11 + 9 = 20
2 + 0 = 2
What if they are higher than 20?
12:09
1 + 2 + 0 + 9 = 12
12 + 09 = 21
2 + 1 = 3
What about 22
12:10
12+10 = 22, 2+2 = 4
1 + 2 + 1 + 0 = 4
So it seems anything thats 20>21 doesn't work out that way.