A map from 1903 of the United States showing the time zones as they were then established. "The heavy dotted line shows the arbitrary divisions of time in the United States. The plus and minus marks on either side of the meridian lines show whether it is necessary to add or to subtract from the standard time of points east or west of these lines, to arrive at actual, or Greenwich time." - The American Almanac, Year-Book, Cyclopedia and Atlas, 1903.