Calculate IT Relief for Arrears of One time increment received

GConnect online Calculator to estimate Income Tax Relief under Section 89 of IT Act in respect of arrears of pay received in the year 2012-13: I have received arrears of pay this year as a result of one time increment granted by govt this year for employees received increments from Feb-06 to Jun-06 in the pre-revised Scale – Should I have to pay income tax for the same? Is there any exemption?