How will j2 visa impact j1's GC process

Hi there, thank you for providing such insightful information. I have a question which I need some advice on. My husband is on a j1 visa and i have a j2. However, I am not in the US yet as I would like to finish up some work I have in my current residential country and join him in one month time. He is probably going to apply for GC soon (he has been there for 2 months). My arrival period will coincide with his application for I-400, will that affect his GC process? Can he apply for I-400 since he would have been there for 3 months minimum, or does the 3 months minimum only start when I get to the US? Thank you so much.