Desmond is from the past, not the future
We’ve been told that you cannot change the past by going back in time.
We’ve also been told that the rules don’t apply to Desmond. Could it be that the rules actually do apply to Desmond as well – it’s just that Desmond is from the past (way back) rather than the future?
Desmond is not going back in time to change things – he is still jumping between points of time in the future, relative to his own time reference, whatever that might be.
There you go. It’s a crappy theory. Not a very good start… Feel free to improve it!
The hole here seems to be the fact that we already know a great deal of Desmond’s back story and he’s roughly the same age as the rest of the losties, mid 40’s. So if he was from the past and moved forward to say, 1977, he would have still been a little kid. I think it is possible that Desmond is from the island circa 1977, and that may be part of why Widmore wants him to stay away from Penny.