I think a lot of the "youngsters" on the board may not know the history of clocks. You old timers feel free to jump in to fill in more information.
In my early days of radio ( in high school) I never saw a format clock. The only thing I knew was to follow the log. Hit the news on the top of the hour, play a song (any song), talk, play commercials, play any song that was available to play, talk, more spots, talk some more, community billboard maybe a jingle play a song that I liked or a song I knew the owner liked., etc.
I believe that Bill Drake pretty much changed all that. I think he was the first to figure out the modern Arbitron book. He came up with sweeping the quarter hours with a hot hit to get credit for each quarter hour. He came up with the idea of 20/20 news. Up to that time stations had news mostly on the hour and half hour- with the exception of some stations like WABC with news at :55 (news five minutes sooner on WABC New York) and KB with news on :15 & :45 but still killing the quarter hour. The first time I heard the hiding of spots in the hour was while in college in the early 70's and listening to WOLF. Coming out of the :50 break with: now another 12 in a row, AND THE HITS JUST KEEP ON COMING!
The first station I worked at that had a clock was WTLB. At the time they were converting over to a quasi Drake format. The clocks were in a notebook over the board. There was a separate clock for AM & PM drives, midday's, nights, and weekends. Each song on the clock were color coded -red was a hot hit corresponding to a 45 with a red circle sticker on it, blue was the second rotation, green was recurrent and yellow was for an oldie. It was explained to me then the importance of sweeping every quarter hour. Get credit for the quarter hour by keeping the listener there for another five minutes. I know most stations follow this strategy now days but I wonder how many of the on air people know why or where it came from.