Hi,
@seedgrumble.
To amplify what
@mcogilvie has already written, the standard advice given is that if something is due
by a certain date it goes in your lists. If it can only be done
on a certain date, it goes in your calendar, either as a time-specific activity or an activity that is date- but not time-specific (most calendars, electronic and paper, allow you to include the latter in some fashion).
If you use a list manager that allows it, you can put due dates on your next actions. If not, they can go in your calendar.
The idea that next actions should never have a due date is a common misconception. What David Allen is advising against is setting fake due dates to motivate yourself. The idea is that if you do that, you'll go numb to the whole concept of deadlines and possibly miss ones that are real.
(Don't feel bad about having misconceptions about GTD. It happens to just about everyone. I've had more than a few misunderstandings about the material myself. There's a lot of information in the GTD book and it flies in the face of what passes for "common sense" when it comes to "time management.")
Examples of real due dates include a deadline for submitting your tax return, a due date for handing in homework if you're in school, or a deadline set by your boss for accomplishing something. Fake due dates would be ones where you decide "I
should accomplish this by such-and-such a date because it would be preferable". I think your self-imposed deadline for buying wallpaper would qualify as a fake due date.
Now, you don't have to do things the way David Allen recommends. If you would feel better setting some "fake" due dates, by all means, do so. They're your lists. They won't, y'know, explode or anything if you do things differently than is presented in the book. (At least I hope they won't. GTD is tool-agnostic. But if your list manager is incendiary to the point of being capable of a literal explosion, I feel safe in saying you need a different list manager. They should not explode. It's dangerous. And it won't help you get your work done, unless you're in the building demolition business or something similar.)
If you're on the fence about it, though, or you've already committed to going "all in" with GTD as David Allen presents, I'd suggest trying it the way he suggests and limit due dates in your system to those that are real. What's the worst that can happen? Maybe over time you decide you don't like doing it that way. Well, then, at that point you can always try something else.