FWIW, only one of my three kids really had the concept of adding time like this take hold easily. I was equally surprised and really don't know why! Sometimes I wonder though if it's possibly related to living in a world of digital clocks?
I think maybe turning time into anything else, e.g. angle or numbers, just is fundamentally a hard concept for some (many? most?) people. My DS took a surprisingly long time to learn to read an analogue clock face reliably, and I might blame it on digital clocks, if it weren't for the fact that my only memory of finding anything hard in the first few years of school involved page after page of clock faces to interpret!
