I have both an iPhone and an iPad -- and I think Shelley does as well, as I'm sure she has indicated thus in many of her posts. Contrary to what you're saying: start time/date settings are identical on both devices.
I think I know where you're confused. You have the selection page for start date on your iPhone set to show the wheel view, which allows allows you to change the start time alongside the start date. And you have the selection page for start time on your iPad set to show the calendar view, which does NOT allow you to change the start time -- but there is always, always a start time assigned alongside a start date, and the default is 12:00 am
To switch between the two views for selecting start date & time, tap the little icon near the lower right corner.