(35) Plan year. A plan year must be twelve consecutive months, unless a short plan year of less than twelve consecutive months is permitted for a valid business purpose. A plan year is permitted to begin on any day of a year and must end on the preceding day in the immediately following year (for example, a plan year that begins on October 15, 2015, must end on October 14, 2016). A calendar year plan year is a period of twelve consecutive months beginning on January 1 and ending on December 31 of the same calendar year. Once established, a plan year is effective for the first plan year and for all subsequent plan years, unless changed, provided that such change will only be recognized if made for a valid business purpose. A change in the plan year is not permitted if a principal purpose of the change in plan year is to circumvent the rules of section 4980H or these regulations.