The events that occur in "Brave New World" ARE in chronological order, but Huxley breaks them up into several different sections so that he can follow different storylines all at once.  This is a technique that we should be pretty familiar with, because movies and television enlist this strategy all of the time.  We will see a scene with one group of people in the movie fighting a battle, and then the movie will cut away to show the leaders of the battle on the top of the hill discussing the action.  In this novel, Huxley does the same thing--he cuts between Bernard and what he is doing at the moment, to Lenina and Henry, to Mustapha.  All of the characters are doing different things, all at  the same time.  In one sequence Lenina is talking with Fanny, then it cuts to Mustapha giving a tour of the hatching facilities, then it cuts to Bernard going about his business, and it goes back and forth between these three scenes pretty rapidly. It can get a bit confusing at times, in writing, without clear markations of when you are switching storylines.  In movies it's a bit simpler because we can clearly see that the scene has changed.

So, for the most part, the plot is chronoligical, if patchy.  We do get flashbacks when John tells his life growing up, and when we get the story of how Linda was left behind, but other than that, the tale is told all in the same time period.  I hope that those thoughts help; good luck!