Yes, actually `setFireDate:` is the method that I need in my project and it is the main reason of this pull request. I have a timer with 5sec interval, and occasionally I need to set the following firing in 0.5sec and then again have a 5sec interval.
Of course, recreating timer is an option, but I believe that it is more convenient and straightforward to use a NSTimer-like API, especially considering that `dispatch_source_set_timer` supports multiple calling for adjusting time interval.
What particular unit tests do you mean? Checking for thread-safe? Or for correct fireDate adjusting?