I bred an Eevee. I have given it at least 5 vitamins and leveled it up to level 30. I'm playing Emerald. The internal battery has run dry, but the clock says 11:59 am. The person in Verdandurf Town said my Eevee is happy. Why won't it evolve into Umbreon?
When the battery runs dry in Ruby, Sapphire, or Emerald no time based events will work AT ALL. This includes, but is not limited to evolving Eevee through happiness due to the need for a WORKING clock mechanic
In Pokémon Emerald, Eevee doesn't evolve into Umbreon through happiness, it evolves through having a moon shard in your bag in Pokémon XD only.