You may plan to start writing at some future date... but that is a dangerous prospect to consider. The danger in starting some day, is that it means "not today." Of course that isn't likely news to anyone, but it is something that bears consideration. I once saw a shirt that said "Procrastinators unite tomorrow!" It really made me laugh, because that is never going to actually happen. If you have wanted to start writing a book, tomorrow, or next week, is not the time to start. RIGHT NOW is the time to start. Here is a question to ask yourself when you are considering when to start writing, "When is now, the best time to start?"
Don't wait to start, even if all you do is write down a title, a first sentence of your prologue, or a sentence of your outline. The quantity isn't so important as the simple act of beginning. There is a single common trait every single successful person on this earth shares in common. I'm not talking about a trait that most share, but a single one that EVERY SINGLE SUCCESS shares. "They started." So don't wait, start today. After all, now is always the best time to start.