qué significa ‘hace un tiempo estupento´
it means 'have a good time' or 'the weather is good'?

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amelia, 
un tiempo estupendo significa que el día esta soleado, luminoso,agradable, maravilloso, que es un día que alegra nuestras vidas

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the weather is nice
天气很好

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Hello Amelia,

"Estupendo" ( not estupento) means marvelous , splendid or great not just good.

"hacer" here means "it is "

The sentence means:
"The weather is great (splendid)"
" It is a great ( splendid)weather" or

" The weather is good " would be literally 
" hace buen tiempo".