I just ran a quick test, and formulas work fine with the new timeline view. One critical point to remember about this new view is that the start and end fields need to have the same options in their settings (e.g. time vs no time, GMT vs not GMT). As long as they’re both configured the same way, it should work.
Re: the 5 pm default, are you saying that you’d like the ability to override that time in certain cases, or do you always want it 5 pm no matter what?