within() timing guarantee

[lexuscrossover]

Hi,

I am implementing some timeout logic with Facebook folly library's within. I have the following logic:

    CallThatReturnsAFuture().within(std::chrono::milliseconds(10)).onError([](const folly::TimedOut& e){
      //handle the error
    });

I noticed that sometimes, when the CallThatReturnsAFuture() can return in 12 milliseconds, let's say; but the onError lambda does not get invoked. I am wondering what the timing guarantee is for within().

Thank you very much for your help!