Well they can TRY at least. This is how it works if your day is for instance 17th of the month add 7-10 days to that. Usually they try to set the date the very next week.
On a separate note as u mentioned I know a friend in Pakistan who once said that she adjusts her period days if she has to travel or if someone's wedding is coming. To get 'em early she would eat food that is generally hot & do the opposite to delay her periods. I know I go crazy on coffee sometimes & it works for me.