Please see the beginning of the episode Mr. Monk and the Rapper for reference to the clock.
Disclaimer: I do not own Monk.
The clock resting on the mantel had been bothering Mr. Monk more and more recently. Two seconds. It was off by two seconds, and that knowledge was beginning to leak into his dreams, turning them into nightmares. He really needed Natalie's help in correcting that. As luck would have it, she was going to be over to return his levels soon. He would bring it up to her then.
Not long after that thought had crossed his mind, Natalie walked in. When he looked in her direction, he saw that the bag she was carrying obviously held more than his three levels, so, due to his curious nature, he asked, "What's all this?"
She looked at him smugly and replied, "It's your levels as they were always meant to be; in a perfectly even group of ten." She would admit, if only to herself, seeing Mr. Monk's shocked expression was the highlight of her day. He had obviously recognized his mistake.
The satisfaction of being able to prove her point was well worth investing extra money into buying the additional levels. She still didn't know how he could miss the fact that three was an odd number. He never did anything in odd numbers!
Mr. Monk began to mentally reprimand himself. How? How had he managed to miss this? It was so blatantly obvious, and yet he had completely overlooked it. This was not acceptable.
It was then he smiled. Having a clock off by two seconds was equally unacceptable. It would ruin any form of countdowns, such as the ten second countdown used to usher in the New Year. Now there was no way for Natalie to refuse to assist him in correcting that clock, not that he believed she would have anyways.
Oh how he loved numerical facts, especially when they worked in his favor.