Either way, they usually publish on time.
Does your magazine tell you "Our next issue will be published May 20th?" No, not exactly, but at the same time each month, a new issue is on the shelves of your favorite bookstore. Complete with advertisers, nice printing, good content and at least a little time devoted to proofreading and checking. Why does this work for a magazine? Because they have something called internal deadlines. Actually, it's not because they *have* internal deadlines, but it's because they *follow* those deadlines. The advertisers know they have to have their final artwork in by a certain date or the publication goes to print without them. Writers know they have to have their text ready, proof-read and fact-checked by the time the layout process begins. The art director knows that the illustrations and photographs have to be ready, edited and done by the time the layout process starts. Most of all: they DO IT. The writers HAVE their items read and edited on time. The art director HAS all the illustrations and photos ready to go on time. The advertisers probably had their work in EARLY. There is an established layout process, possibly by spread, possibly by whole shebang, where the group of designers work with everyone to literally create the magazine.
How do newspapers do it? They have to publish EVERY DAY! What would happen if some very important writer asked a morning newspaper to hold production until the afternoon because his story was SO important and SO critical, but he didn't have time to actually write it yet?
What if a magazine photographer, at the last minute before going to print, holds the production because the headshot of the bigwig is not QUITE right.
How can a publication be on time if the internal deadlines are COMPLETELY IGNORED?
It can't.