What is the ruling regarding a muezzin's knowledge of prayer times?

Primary text

It is recommended that the muezzin be knowledgeable of the prayer times so that he can accurately call the Adhan at the beginning of the appointed time. If he is ignorant of the times and makes a mistake, his Adhan remains valid. This validity is established analogously, as the Adhan of a blind person, who is more likely to err regarding time, is valid.