Tax is calculated on your total income earned or received during the year. If your total income includes any past dues paid in the current year, you may be worried about paying a higher tax on such arrears (usually tax rates have gone up over the years).
To save you from any additional burden of tax due to delay in receiving income, the tax laws allow a relief under section 89(1).
In simple words, you do not pay more taxes if there was a delay in payment to you and you were in a lower tax bracket for the year you received the money.
According to Section 89, if you get salary in arrears or advance in a financial year due to which your total income for the year increases, which in turn increases your taxable income, you can claim for relief under Section 89.