Hi, Ilko—Yes, both sentences are correct. It would be good if you learned to number your example sentences and to write more than a line or two in a post. Notice that a comma should be used after the introductory "by"-phrase.
(1) By the time I turned around, the door opened. (2) By the time I turned around, the door had opened.
Sentence (1) indicates that the door, which had been closed for a while, opened right when you turned around. Sentence (2) means that the door opened before you turned around.
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