Final answer:
The proverb 'opportunity knocks only once' suggests that chances for success are rare, and a person should be ready to seize them. The mention of luck regarding knowledge of time illustrates that accidental correctness isn't true knowledge.
Step-by-step explanation:
The old proverb states that 'opportunity knocks only once'. This phrase suggests that chances for success or advancement can be rare and may not present themselves frequently, so it is important to be alert and ready to seize them when they do arise. The conclusion that it is only a matter of luck that the man knows the correct time implies that his knowledge is accidental and not based on substantiated facts, thus he does not truly 'know' it. This is an example of how the concept of luck can be intertwined with opportunities and knowledge.