Reference: Fataawa Ramadhaan – Volume 1, Page 212, Fatwa No.155
Question: We live in a land in which the sun does not set until 9:30pm or 10:00pm, so when should we break our fast?
Response: Break your fast when the sun has set. So long as you have a night and day in 24 hours, it is obligatory that you fast even if the day is long.