Translation:When did this accident happen?
It appears that this is a "是……的" sentence, but it's not clear to me why "在“ is being used in conjunction with "什麽時候".
I don't think so. 什么时候 already means "at what time" by itself, so no need to add 在 before.
I am not sure why 在 is there, but i assume it is used here to show that the action was in progress (same meaning as 正在). So a more literal translation of this sentence would be: "At what time was this accident happening?". But again, this is just a hypothesis. I hope someone will give us the answer.
Just asked a Chinese friend, she said with or without 在 the meaning and connotation is exactly the same. So I guess it's just optional