Some of the comments here claim that we already have pretty long lived and robust software. While this may be true, I don't think that's what DARPA is looking for. I believe that they are looking something that could keep an interstellar probe running for few hundred years, or something like a remote base somewhere in space. Something that could detect and fix any issues with the software and hardware. But it's not only about reliability. Think about a Moon base, where the software keeps running for decades, and once in a while new module/hardware is added to it. MoonOS would have to detect new addition and reconfigure itself to work with it. Maybe it could even decide to move some critical parts of itself to the new hardware. It would do it without any input from humans.