This week we were supposed to write a film review in the school. I used a phrase in mine, which was : "in a time, when"

I saw/heard it many times and it was a big surprise when the teacher said it was incorrect and I was to use "during time" instead. I tried to argue with him (not sure it was a clever thing to do, actually), since the net is flooded with reviews using "in a time when", but he said it was all wrong. But surely the Empire, too, wouldn't write incorrectly? Anyway I just would like to hear more from you about this.

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Comments  (Page 2) 
"in a time when" or "at a time when" are both perfectly fine. I don't understand your teacher's insistence on "during", especially "during time" (if you've quoted it correctly) instead of "during a time (when)".