He must make up the fast without expiation, unlike one who breaks their fast merely due to doubt about the sunset; because the default is that the day continues, and mere doubt is not sufficient to exempt from expiation. If the situation is not clarified, he does not have to make up the fast. However, if it becomes clear to him that the sun has not set, he must make it up. Refer to Badai' al-Sanai' 2: 100; as narrated by Asma bint Abu Bakr, may Allah be pleased with them, who said: 'We broke our fast during the time of the Prophet, peace be upon him, on a cloudy day, then the sun appeared. It was said to Hisham: So they ordered to make up the fast, he said: There must be a make-up.' In Sahih al-Bukhari 2: 692.