The illustration given by Mr.S K Bandyopadhyay , is perfectly correct arithmetically. However, view expressed by Mr. Madhu, is a standard practice followed in many companies.
I would also like to mention here, few big companies go a step further. For example, one of the leading Auto majors in India and now an MNC company, have a system of retirement dates, as the last date of every quarter, viz 31st March, 30th June, 30th Sept, and 31st Dec every year. Those staff whose retirement age falls between 1st Jan - 31st March, shall be retiring from service on 31st March and likewise in remaining 3 quarters.