Going back a bit in this thread:
O’clock,” I thought, simply meant “of the clock,” as in time of the clock, or “time of day.”
Q: “What time is it?”
A: “It is nine ‘o’[f the ]clock.’”
Doesn’t “o’clock"=time of day as read by a reader of the clock?
The OED’s first citation for of the clock is 1386, from the Canterbury Tales: “Ten of the clokke it was tho as I gesse.”
Now, in his fascinating book The Time Traveller’s Guide to Medieval England: A Handbook for Visitors to the Fourteenth Century, Ian Mortimer remarks that at the beginning of the 14th century there was only a single experimental clock mechanism in the whole of England (at St Albans), so everyone told the time by the sun, either by eye or using an astrolabe. An ‘hour’ was defined as one-twelfth of the natural daylight or night, whatever the length of said daylight or night, and therefore was a constantly-varying span of time. But in the 1350s clocks started to be introduced, and by the end of the century most palaces, castles, religious foundations and prosperous parish churches had one. As they told time not by moving pointers round a dial but by striking bells (hence their name), a fair slice of the population - the urbanites, the religious, and the elite - could now function by clock time. But as the clock hour is, by its nature, a fixed span of time, except at the equinoxes it was never quite the same as the sun hour, and near the solstice it could be radically different. Therefore, when Chaucer was writing, if you wanted to make clear when you meant you had to specify either ‘ten of the clock’ or ‘ten by the sun’.