Hi Abhishek,
U may have to refer to the rent agreement for this. TDS should be deducted at the time of credit or payment whichever is first. The question is when u'r rent becomes payable whether on tthe monthly basis, bimonthly, half yearly, quarterly, or some other basis.
if it is payable the monthly basis, u should have to deduct the TDS on the monthly basis and in that case u'r TDS rate would be 15% for april 2009 to sept 2009 and 10% thereafter. Sometimes agreement specifies that Rent should become payable at the 10th day of each succedding month, if it is for the month of september 2009, credit given in the books to landlord on the last day of month and thus TDS would be deducted at 15% even if the payment due date is 10th of october. The question here arises when is u'r liability to pay rent arises, it is not arises on the 10th day- it's only a due date. If is the date of last day of the month (in case rent becomes payable on monthly basis)
U doesn't even have to refer to any case law. it is much more clear in the law itself.
Hope this solves u'r query.
Regards,
Manoj