I often find myself underestimating the time it takes to accomplish certain tasks. E.g. I think "ok, I'll send a couple of emails in 0.5 hours and move on to the bulk of the work" and it turns out it takes more like 5 hours, and not because I'm procrastinating; it actually takes me that much time to do it.

What about time-management? As far as I understand, GTD, while often described as time-management system, actually doesn't help you manage time. As far as I understand Allen (I have one chapter left, and after that I plan to read the notes I downloaded in lieu of re-reading, because I still don't understand what my GTD should consist of) his position can be described as

When you will have an external system containing all your projects and next tasks, you will be able to make decisions so efficient, that you won't need to manage time"

1. Do I understand it right?

2. Do you feel you need a separate time management system in addition to GTD?