I'm trying to remember the title of a book I read in fifth or sixth grade, meaning it would have to have been published in the mid-90s or earlier. It was a young adult or maybe even children's novel. My memory of the plot is very hazy, but it went something like this:
Two teenage boys are involved in some sort of computer simulation game, and one of them figures out a way to time travel within this game. Subsequently, he realizes that his method of time travel will work in real life as well. Someone he knows (his girlfriend's father?) wants to buy or publish the time travel method, but the kid realizes that this will result in all sorts of time travel shenanigans and decides against it. Unfortunately, the buyer/publisher already knows about the method (which I believe was some sort of mathematical formula) and wants to go ahead and publish it against the kid's wishes.
Now here's the one detail I remember very clearly: the kids figure out how to thwart their antagonist when the inventor of the time travel formula realizes that a simplification he'd made to the formula renders it ineffective for anti-matter. The kids confront the antagonist with this challenge at some sort of public meeting and thereby save the day.
I realize that's all extremely vague, but if anyone has any idea what book I'm talking about, I'd greatly appreciate knowing the title. Thanks.