Factual error: After returning to the Dursley's after the dementor attack It is dark, this shows again when Vernon puts Dudley in the car. However, the howler from Mathila Hopkirk says that at 6:23pm Harry performed illegal magic. As the side story shows it is a hot summer, it wouldn't get dark until way past 8pm. (00:04:53 - 00:07:20)
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Suggested correction: Fairly certain the film was clear that the reason it got dark was because of the dementors, they have the ability to change the weather to depressive darkness.