According to The Time-traveller's Guide to Medieval England 'of the clock' was used to describe time when it was being sliced in 24 equal parts (hours) of the day.

It was used to differentiate the practice, used equally as much, of using solar time, whereby the 7th hour would shift in actual time, however would always be 7/12ths of a solar day after sunrise, and the length of an hour would increase in summer and decrease in winter.