Date earnings are counted during retirement year

Your earnings are counted for all months of your taxable year even though you may not be entitled to benefits during all months of that year. However, there is a special rule that applies to earnings for one year.

Please see these links :

http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/335/

http://www.socialsecurity.gov/retire2/rule.htm