If you are asking how your Eevee evolves when battling, it evolves into either Espeon or Umbreon when it has high friendship (200+) and evolves under these timeframes.
Day time is 4:00 AM - 5:59 PM and night time is 6:00 PM - 3:59 AM.
If you are asking how to encounter an Espeon or an Umbreon when SOS chaining, then this is my answer. When you SOS chain for Eevee at nighttime/daytime, you'll have a chance to encounter Umbreon/Espeon on either Route 4 or Route 6.
Also, if you go through the Ultra Wormhole during the Post-game with either Solgaleo or Lunala in your party, you can "temporarily" change the day into the night (Sun) or the night into the day (Moon).
Source: Knowledge of in-game day/night cycle and Post-game