How does the Getting Things Done (GTD) system handle the concept of task due dates? As I understand it, tasks that must be done on a certain day go on the calendar, but what about tasks that have to be done by a certain day? How are they categorized and how is the due date managed?
The due date is simply a meta property of your task, just like time and energy estimation.
In NirvanaHQ, for example, you can set the due date so it shows a grey box at the end if it's still to happen and a red box if it's past the due date. And it also allows you to sort the current view by due dates.
In the current version, it also has the option to schedule a task. But they are removing that in the newer version because GTD isn't meant to be scheduling tasks for later, but rather performing the task(s) now...
There are different approaches for tasks with deadlines but they can still go to the calendar.
Split the task
Split the the task in smaller tasks to be done in different dates. When you get the reminder of the 5th task you may remember the 4th still isn't done and make an effort to keep up.
Split the time
If you don't want to bother splitting a task, you can also set a number of hours a day you're going to work on it.
The way I saw it done in a GTD webinar was like this:
10/21 Install backup script 11/1
10/21 Write down ideas re: taking piano lessons
The first column is the date in which you wrote down the task. It is optional, but in my case for example it has been pretty useful, because it allows you to see how long has been a certain task hanging around, otherwise things kind of float in a void... (and when you see a task has remained in your list for maybe a month, you can perhaps renegotiate it, decide that you don't need to do it anymore, etc...)
The second column is the task itself
The third column is for due date if there is any. In my example, the first task does have a due date, but the second doesn't.
Hope it helps.
For items that must be done by a certain date, but that do not comply with a classic "appointment" I have two ways of handling them.
The first one is, if you need a block of dedicated time to perform a certain task, just block this in the calender. This way I will be noticed of it and ensure I have a dedicated time frame where I can work on it. I normally ensure this is on Monday morning or Friday, where I can schedule time in my home office so that no interruptions prohibit me from performing the task at hand.
The second one is that I put a all day task on top of my (Outlook) calender. This ensures that on that day I check those tasks and perform them. If it is something very important, for example preparing a performance review which must be done one day before the meeting itself I will put this on the day before. Going through these items is of course a must for the weekly review. As soon as I finished the task I leave it there, but prefix it with "DONE // " to ensure I have ticked it off.