It depends on the circumstances.
Write a book on my own? No prob. Start "write" away
Write because that book is halfway done, but it's not expected to be done for another month or so. Nope. Not happening.
Same "task" different circumstances, different start times.
But I do have ADHD, although I've had psychiatrists/therapists tell me the bipolar-schizophrenic spectrum brains are so chaotic they might as well.