The aforesaid proviso makes it clear that the unspent amount should be deposited before the due date of filing of return under section 139(1) of the I T act. As you stated in your case , rightly so by 31/7/2008 . However, you will be pleasantly surprised to know that the higher legal authorities have interpreted section 54F(4) to read that the last date is not "due date for filing return u/s 139(1) but can be any other sub-sections under section 139.
Therefore an assessee, as per these decision of Tribunal , can utilise the gain by the date of even late return filing u/s 139(4) of the I T Act which increase time by at least one and half year because the last date by which a return is to be filed is one year from the end of assessment year .

The Income Tax Tribunal , Banglore ,Bemch B held in case Nipun Mehrotra vs ACIT,Cir (12), 110 ITD 520 [2008] in favour of assessee that the section 139 mentioned in subsection 54F(4) can not be interpreted to mean only 139(1) but can also mean section 139(4) .
The facts of the case was as under :

• The assessee earned long term capital gains on shares for Asst Yr 2000-01.
• He paid a part of sums after due date of filing of return i.e 31/7/2000 for Asst Yr 2000-01.
• The A.O disallowed the exemption Rs 2,10,833 u/s 54F on the ground that amount was not utilized /invested within the specified period i.e 31/7/2000.

CIT(A) confirmed the order of A.O . The assessee approached Tribunal which accepted the contention of the assessee that he invested more than capital gains in new residential house within time allowed for filing return u/s 139(4) i.e late return. As per assessee , section 54F(4) mention only section 139 and not it can not be interpreted to mean 139(1) but one can also interpret 139(4).

The Tribunal relied on the decision of Gauhati High Court in case of CIT vs Rajesh Kumar Jalan [2006] 286 ITR 274 . In that case the Hon’ble High Court held that section 139 mentioned in section 54F (4) will not only include section 139(1) but will also include all subsection of section 139.

The Tribunal ,taking cognizance of the decision of the Gauhati High Court , find similarity in section 54(2) and 54F(4) of the I T Act.It held as under:

Section 54(2) was substituted by Finance Act 1987 . The scope and effect of amendments were elaborated vide Circular No 495 dated 22/10/1987 . Sections 54(2) and 54F(4) were introduced to dispense with rectification of assessment in case the taxpayer fails to acquire the corresponding new asset . Hence ,if the new asset is acquired before the date of filing of the return under section 139 then the assessee can file sucj return and there will be no need of rectification. Thus the interpretation , which has been placed by the learned Gauhati High Court , is in accordance with the legislative intent of introducing sections 54(2) and 54F(4) .

Taxworry’s view
I am surprised on the decision because section 54F (4) also mention the word 139(1) . Read the provision u/s 54F(4) once again [note the words in red]

54(4) The amount of the net consideration which is not appropriated by the assessee towards the purchase of the new asset made within one year before the date on which the transfer of the original asset took place, or which is not utilised by him for the purchase or construction of the new asset before the date of furnishing the return of income under section 139, shall be deposited by him before furnishing such return [such deposit being made in any case not later than the due date applicable in the case of the assessee for furnishing the return of income under sub-section (1) of section 139 in an account in any such bank

The said section clearly provides that the unspent amount can not be deposited in capital gains account scheme later than the due date prescribed u/s 139(1) which is July for individuals. Therefore in the foregoing section if the law makers mention 139 , in my opinion , it means 139(1) only and not 139(4) , otherwise whole provision u/s 54F will become self-contradictory.

But anyway, if there is relief from court , use it when required!