The clock exercise is a standard diagnostic tool. A clock face is an easily recognized and near universal object that we are taught to understand at a very early age. The test is easy to administer and the mental processing errors are immediately obvious.
It sounds like an easy task. It's actually quite a complex task. The individual has to be able to imagine the clock face, recall the numbers and their positions on the clock, keep the requested time in mind, and correctly render that time on the drawn clock face.
Lyn has never been able to draw a complete clock face. The test would not have been a useful tool for her because of this life-long inability. She used to understand it though and could read the time from it. I chuckle as I recall how she would try to manipulate people, particularly men, into giving her attention by convincing them she didn't know how to tell the time. She no longer can tell the time from an analog clock. She can still read the time from a digital clock.
It's absolutely worth listening to because it highlights why individuals with dementia struggle with drawing a clock. The first link above will let you listen to the story without having to sit in your car to hear the whole thing.