Espeon and Umbreon

The change is visible in the app, for me is a little over 6 pm from day to night, it's pretty noticeable, to be sure, I'd try somewhere between 9 am and 2 pm for day and 8 pm and 2 pm for night.

I think the cutoff is when the app background itself changes from bright (day) to dark (night). I read somewhere that this more or less follows sunrise/sunset patterns in your location, so there's no constant cutoff - it'll change throughout the year along with the length of a day.