About the 0 you've got, think about what is 24 modulus 24 (i.e. the remainder of the division of 24 by 24). And for the 1 missing, you would need a 6 in your input to get the 1 in the output, and you don't have it.

Update: BTW, the difference between 0 and 24 is also a matter of cultural differences across nations. In many countries, the time one hour after 23 pm is 0 am, in others it is 24 am or pm. But 00:00 or 24:00 are in effect the same time.