There are do dates, and there are due dates. There are no doo dates, but the former two will suffice. There is a distinct difference, though, between the two.
You are most likely more familiar with due dates; they’re normally dates a task must be done by. However, they don’t strictly have to be. False due dates can help you, say, when you have a big project that you only have one due date for. If you separate the project and add a different due date for each section, it becomes much easier to avoid the dreaded last-minute scramble.
Do dates, also known as Start dates, are just as powerful, if not more so. These are the days you want to do something. A basic principle of GTD is to not have to think about tasks you’re not working on. However, you want to still have enough time to do the task before it pops up into your view as due tomorrow. Whenever you have a due date it’s usually helpful to also have a do date that will give you a heads-up before your deadline.
Do dates don’t require due dates, though. A task like “Read Twilight” could be useful if you want to make sure you remember to do it, but have a stack of other books piled up, unread, on your shelf. You can postpone it until you think you won’t have any more books to read, but still have a reminder to do it.
The opposite goes for a gift card that will expire; you can use it at any time before the expiration, but not necessarily have to begin on a certain date. In this case, a task like “Buy Twilight with Borders gift card” would have a due date but not a do date.
I hope this helps you stop procrastinating, and allows you to more effectively use Do and Due dates. Many apps, including Things I believe, allows you to add a Do date only by setting how long before the Due date the task will appear in your inbox. As I said, a Due date pairs up nicely with a Do date, so this shouldn’t be a major limitation to your GTDing.
And speaking of which, happy GTDing!