But that's why it's tricky... more times than not, it's an excuse to put something off, and often a small white lie to yourself in order to make you feel good about yourself. I mean... life is full of surprises - how could you know for sure whether you can do it successfully until you try?
The time limit doesn't necessarily mean that you're going to get it done by a certain time, it can be that you are going to allocate your time to the challenge as your first priority once your basic life needs are met. That is definitely a specific time, and not any time.
However, let's just use the method of picking a set time for now, and creating a sense of urgency - that this is an important thing that must be done by a certain time. It's important to get in the mindset that it's a crisis and if it doesn't get done, bad things will happen, or alternately good things won't happen.
There is a "sweet spot" in terms of picking a time. It's important to not go overboard and create so much pressure and stress with a short deadline that productivity is impacted as well as quality of life, but the goal needs to carry enough threat of penalties, or desire for rewards, that we definitely start moving towards getting it completed right away. How can we tell if we've succeeded? It's simple, it's all in the emotions. Notice the word "motion" in emotion. With sufficient emotion, we start the motion towards completing a task. It takes some practice, but if you do it right you will literally feel that you are holding yourself back from completing the task, and will just need to stop holding yourself back in order for progress to start.
So when picking a time, it's important to find a deadline that will definitely motivate us to start working on it right away (or to start at a very specific time) but won't be impossible to achieve... to pick the right spot where you have motivation to start, but that limits exposure to additional stress beyond what is needed to accomplish the task.
You need to sincerely believe that something must get done under some sort of time limit for life to get better, to put it all in perspective. This is essentially a value exercise and resetting your values is covered in the book.