Bonsoir Brian, C'est drôle ! I had a very similar question in one of my French classes a few years back and the prof explained it thusly, to use J'arrive as a future statement she considered that to be sometime within the next 24 hours. She further explained the near future statement using aller, i.e. Je vais venir, to be a time frame of 3 to 5 days out. Anything longer than 5 days out she considered as needing the future tense. I am unsure where she had come up with this "time frames", but on a certain level, it kinda makes sense. J'espère que cela vous aiderez. Bonne chance, Ron