Exemptions under Section 54EC are only available on gains from transfer of long-term capital assets.The person must invest a part or the entire capital gains within 6 months from the date of asset transfer.The benefit is available only if the investment is made in the specified long-term assets
When long-term capital assets are transferred, the gains made from the transaction will be called long-term capital gains. This attracts tax as well, termed as long-term capital gains tax or LTCG tax. However, there are certain exemptions available under Section 54, Section 54B, Section 54EC and Section 54F of the Income Tax Act 1961. As of FY 2018-19, exemption under Section 54EC applies only to transfer of long term capital assets specifically land, building or both.
You should invest in NHAI within 6 months from the date of transfer (sale).