Earlier quoted context omitted.
Getting Things Done does not focus on concretely scheduling tasks for specific times and dates, unless I missed a huge piece of that book.
You're correct, in fact, it warns explicitly assinging a datetime unless it _has_ to be done by then. That being said, the biggest reason my GTD practice fails is I rarely spend time getting things done, but instead end up grooming an ever-growing to-do list. Assigning datetimes to trigger reminders is tempting...
Instead, let your list go a little wild, and put the stuff you truly don't need to do in that "Someday" pile and don't look at it until you've gotten a few projects done from the pile you've determined is important.