JdeBPYou have 5 bits for the hour of the day in a clock chip register. Do you just use a binary representation from 0x2x0_0001 to 0x2x1_0111?
No. That would be too easy. Programmers might use your hardware, and that would never do.
So you make bit 4 mean add ten to the value in the lower four bits, and document hour 23 as being hour 13+10.
And for kicks you do that for the 5 bit month number register, too. Because 4 bits storing just 0x2x0001 to 0x2x1100 would just let those pesky programmers in, again.
What on Earth were these chip designers on? BCD, I could understand. Even giving midnight the value 8 I could understand. But this scheme is just mad.
https://files.seeedstudio.com/wiki/Pi_RTC-DS1307/res/DS1307.pdf