Har Cheez Ka Waqt Muqarar!
IMHO we pick the time also, like I decided to comment here at 9:34 PM EST.
I don’t think there is a fixed time for anything but it may seem like it because Allah is aware of past, present and future. Also everything that will ever happen is already written down. So, having knowledge of future doesn’t mean it is fixed, but it will happen exactly as it is already written.