A. The United States uses the Gregorian calendar which measures out a calendar year that is just slightly longer than the solar year (about 25 seconds a year longer). Pope Gregory XII called to reform the Julian calendar in the late 16th century, which was established by Julius Caesar in 62 B.C. It was 11 minutes and 14 seconds longer than the solar year and by 1580 the difference between the calendar and seasons had grown to 10 days and would eventually lead to religious holidays falling in the wrong season. The Gregorian calendar was first adopted by Italy, France, Spain, Portugal, and Luxemburg in 1582, by Great Britain and the colonies in 1873, and took Greece until 1923 to adopt it. Today the calendar is used worldwide for nearly all non-religious purposes.
Source: Holidays and Anniversaries of the World(1998), edited by Beth A. Baker and James Jespersen and "Gregorian calendar." World Book Online Reference Center. 2006. World Book, Inc. 1 Mar. 2006