Eevee doesn't evolve at a level. It evolves by stones. The evolutions are:
Jolteon-Thunder stone
Vaporeon-Water stone
Flareon-Fire stone
Glaceon-near the rock in Snowpoint city
Leafeon-leaf stone or near the rock in Eterna Forest
The last one, i don't know
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In Pokemon Diamond, Pearl, and Platinum, if you want to evolve your Evee to a Leafeon you go to the Eterna Forest You will have to level up your Evee there and once it levels up by one level, it evolves to a Leafeon. If you want a Glaceon, you level up eevee on route 217. To get a Flareon, you give Evee a Fire stone. For Vaporeon, you give it a Water stone. And for a Jolteon, you need a Thunder stone.
i think you are supposed to make it happy and level it up in the daytime.
there is no specific time but if you use a stone lv. 20 and up
Level up an Eevee with full happiness at nighttime (after 6pm)
To evolve your Evee into Leafeon, get a leaf stone, and use it on Evee, and it will evolve into Leafeon! Have fun with Leafeon! -Garrett