I think it year 1986-1999. My younger sister is definitely in Gen-Z and she was born in 2000. I met a 34 year old guy and he fit on being a millennial trait.
I my opinion you can't put a specific year for when Millenials stop and Gen Z starts. I'd say that generations fade into eachother gradually with each year being further from the previous and closer to the next. I also think that this generations timeline can't be applied everywhere including post-Soviet states