Exemptions under Sections 54, 54B, 54D, 54F, and 54G all share a structural requirement: the capital gain (or, for 54F, the net sale proceeds) has to be reinvested in a specified asset -- typically another residential property -- within a prescribed window, generally up to two years for purchase or three years for construction. When that reinvestment genuinely can't happen before the income tax return for the sale year is due, the Capital Gains Account Scheme, 1988 (CGAS) is the mechanism that keeps the exemption alive.
How CGAS actually works
The unutilised sale proceeds are deposited into a CGAS account at an authorised bank on or before the return's due date; that deposit itself is treated as if the reinvestment had happened, preserving the exemption provisionally. NRIs can open a CGAS account, but it has to be opened under the NRO route -- not every bank branch offers this for non-resident customers, so it's worth confirming availability before the deadline is close.
The deposited funds then have to actually be used for the qualifying purchase or construction within the same overall timeline the exemption itself allows (commonly discussed as roughly two years from the sale for purchase). Miss both the CGAS deposit deadline and the underlying reinvestment window, and the exemption is lost -- there's no second grace period once the ITR due date has passed without a deposit.
The part people get wrong
The most common mistake is treating the CGAS deadline as flexible because the underlying reinvestment window (two or three years) still has time left. It isn't -- the deposit has to happen by the tax return's due date for that year, which is a much nearer deadline than the reinvestment window itself, and missing it forfeits the exemption regardless of how much time remains to actually reinvest.