Specifics of 'Evolve Eevee into Umbreon at night' - 10KM, 2 candy, at night?

If you only want to evolve Umbreon for the Pokédex entry, the easiest way is to rename your Eevee to Tamao and it will 100% evolve into Umbreon without needing any of the other requirements. Note that this only works once. This is possible once-per Eeveelution, see this answer for all possible names that work.

Here are also a couple of other nuggets that will be useful if you want to get an Umbreon the normal way: https://www.reddit.com/r/TheSilphRoad/comments/96o6nu/psa_the_umbreonespeon_method_is_walk_10k_for_two/

• "Night" is when the in-game sky is dark, and in the case of sunny or partly cloudy weather, the weather icon shows a moon instead of a sun. Basically when the game itself goes into "night mode".
• In addition to the OP's message, you must have a valid GPS location when you do the evolve, as this is needed to determine whether it is day or night at your location.

  Note: Basically do not do the evolution if your GPS location is spotty, like if you're underground in a subway or something.

• You can walk for 2 candies and change buddies but you must make the Eevee your buddy again to evolve.

But personally, if you want Umbreon for Pokédex filling purposes, just do the name method. I did this myself a few years back and it worked, but as I said, it only works once.

Another tip is that the evolve button will actually show Umbreon, or a silhouette if you don't have it in your PokeDex, when the Eevee has satisfied all the requirements for evolution. If the evolve button shows a question mark it can only evolve into the three Gen1 options, and as soon as the walking/candy requirement is done, the Eevee will switch between showing Espeon or Umbreon in the evolution button. Which it shows will depend on if the game registers day or night conditions, but will act as an indicator to let you know the result for those specific evolutions.

To answer your questions specifically:

1. The 10km is counted over the lifetime of the Eevee being your buddy, so you can walk it 10km, remove it as your buddy, put it back as your buddy, and it fulfills the requirements.
2. Same as 1.
3. Night changes based on your locality. Usually the switch from day to night happens around 6-7pm, but it will change based on season. It's not exact with official sunrise/sunset times for your area, but is usually within half an hour.