It's not the 24 hour clock. They A) didn't have minutes and 2) they didn't really have any standard time that was recognized outside of, say, a single monastery. One monastery's time wouldn't have necessarily agreed with another's even if they were only a few miles from each other. It'd be like saying we had an occurrence of 12:34:56.78 9/A/BCD in the year we call 3012 because they switched to hexadecimal and customarily kept time down to the hundredth of a second in the year we would call 4004.