What was the family going to do?

Mr. and Mrs. Smith and their children were going to spend their holidays in the country. They had to be at the railway station before 11: 40 in the morning.  "We’ll have to be ready by 11 o’clock, or we’ll be late." At 11: 10, they still weren’t ready yet except Mrs. Smith. She was sitting quietly in a chair in the garden. Her husband and children were surprised that she was not in a hurry until the car arrived. Then Mrs. Smith smiled and said to them, "Well, before I went to bed last night, I moved our clocks and watches 20 minutes ahead!"