I always find apps like this bizarre. How about practicing willpower? The shortest answer between A (wanting it) and B (having it), is to just do it. And when you fail, non-judgmentally self-analyze and try again.
That's actually similar to what my other co-founder told me when I tried to make this! It turned out, however, that our own willpower can be supplemented by tools. For example, sometimes I don't realize I've been on Twitter for 30 minutes - and as soon as I'm made aware of that, I leave. I think often times, it's about awareness rather than being forced by a piece of software.
There are countless examples of products that help people who can't "just do it". Nicotine patches, non-alcoholic alcohol, direct deposit savings programs, exercise buddies, etc. Everyone needs help, everyone has weak points. No shame in using technology to assist you in one of the hundreds of points in your day that require willpower.
You know willpower is a finite resource, right? This is targeted at people for whom the majority of waking hours is spent in front of a screen.