A Rabbi was giving his weekly sermon on Shabbos during davening. After a considerable amount of time had passed and the Rabbi was still speaking, the congregants got bored and started leaving the Shul one by one.
The next Shabbos, the Rabbi asked one of the men who had left, "Why did you leave in the middle of my sermon last week?"
"Well," the man replied, "it was just taking too long."
"How was I supposed to know?" the Rabbi protested. "There's no clock in the Shul."
"But there is a calendar."