Let me explain.
All of my tasks and next actions are entered directly into my calendar for a specifc day or days. It does not go into my next actions list.
I don't have any tasks or next actions without due dates. If I don't put it in my calendar, there is a good chance that it won't get done in a timely manner. For example, I have a task called 'change cell phone setting'. This task is entered into my calendar for this Saturday. This ensures that I get it done on Saturday.
Some tasks, like the one above, can be done sooner. In such cases, when I have free time during the day, I will look at all my outstanding tasks on my calendar for the next few days and decide which one I can do now based on the context/time/energy/priority model.
There are some tasks (like the one above) that can be postponed for a later date because it is not a high priority. For example, I have a task on today's calendar date called 'play with new camera'. If I don't get around to it today, I can move this task to tomorrow's calendar or a future date. I try not to postpone my tasks that often though.
In summary, my calendar can be considered a "holding bin" for my next actions list. Unlike the next actions list (which have no dates associated with each next actions item), all tasks on my calendar are "hard coded" to a specified date. While it is hard coded to a specific date, some tasks can be negotiable (i.e. done earlier or done later). There is also a small subset of tasks that eventually can be delegated to someone else or deleted all together.
I would like comments from other GTDers.