Why are monthly magazines dated a month before they come out?

Question

For example, March issues of magazines will come out at the beginning of Febraury. Anyone know why this is?

Answer 1

I noticed that it's dead annoying when I'm trying to figure out if i've got the latest magazine since the one's i get are usually out half way through the month.

I think its because when they publish it, it is usually edited quite a while before the date it's released (If that makes any sense)