Calendar Year. Calendar year defines the year as designated by the Gregorian calendar in common use as beginning on January 1st and ending on December 31 of that same year. For example, the calendar year 2006 begins on January 1, 2006 and ends at the stroke of midnight on December 31, 2006. A calendar year contains 365 days except in leap years when a calendar year has 366 days. The calendar year is a universal 12-month period as opposed to a fiscal year where the 12-month period is determined by an industry or corporation for purposes of accounting.