From bulbapedia: seasons affect the time of day, which now varies depending on the current season. During spring, morning begins at 5:00 AM and night starts at 8:00 PM; during summer, morning begins as early as 4:00 AM and day lasts until 9:00 PM; during autumn, morning begins at 6:00 AM and night starts at 8:00 PM; and during winter, morning does not begin until 7:00 AM and night starts at 7:00 PM
You have from when morning begins until night begins to evolve your eevee into an espeon. Remember, this is based on your ds's internal clock, not the actual time. Hope this helped.