I agree with Aditya Maheswari, since for interest calculation a part of the month will also be considered as 1 month period. So if it is deducted on 10th September and paid on 9th October, the default is exactly 1 month from the date in which TDS is deducted. If it is deducted before 10th September, even a 2 days extra will be considered as 1 month and 1.5% interest for the extra month has to be borne.
Relevant provisions of Interest Section 201:
“(1A) Without prejudice to the provisions of sub-section (1), if any such person, principal officer or company as is referred to in that sub-section does not deduct the whole or any part of the tax or after deducting fails to pay the tax as required by or under this Act, he or it shall be liable to pay simple interest,—
(i) at one per cent. for every month or part of a month on the amount of such tax from the date on which such tax was deductible to the date on which such tax is deducted; and
(ii) at one and one-half per cent. for every month or part of a month on the amount of such tax from the date on which such tax was deducted to the date on which such tax is actually paid, and such interest shall be paid before furnishing the statement in accordance with the provisions of sub-section (3) of section 200.”.
Your case fall under Section 201 (1A)(ii)- Tax deducted but not remitted