Therefore, "night" in Pokémon Diamond starts when it is nighttime in your real-world location. This will vary depending on where you are in the world and the time of year.
However, there are a few things to note:
* The game has a fixed time zone. It doesn't change with your actual location.
* Nighttime in the game is defined by the in-game clock. It may not be exactly the same as night in your location.
Therefore, the specific time that night starts in Pokémon Diamond will be different for everyone, based on their location and the time of year.