What is the difference between עת and זמן?

QUESTION: I’m trying to understand the difference between עתandזמן, (as in לכל זמן ועת לכל חפץ תחת שמים), and whether the difference is implicit in their respective etymologies. Also in the common expression “בעתו ובזמנו”… What’s the reason for the duplicate? Don’t they mean the same thing?