Returns the number of weeks in a year.
returns the number of weeks in the year in which date lies. A week is considered to start on Monday, and is considered to be in the year if most of its days are in that year.
returns 52, the number of weeks in 2010.
returns 53, the number of weeks in 2009. In 2009, both 1st January and 31st December fall on a Thursday; the weeks containing 1st January and 31st December both have most of their days in 2009, and therefore both are counted.