TDS is requried to be made at the time of Credit or payment whichever is earlier. Once TDS is made it should be mandatorily requried to be made within 7 days from the end of the month in whihc the deduction is made. As TDS is an statutory payment it is reuried to be made within the due date even if the paymetn is not made to teh party. As teh business is carried on by giveing/taking credit, he amount remaining payable to the creditor can be paid within the credit terms as mentioned on the bill.
for the bills provided at the last day of the year, TDS shall be made at that time & should be paid to the Cent. Govt before 30the April as it is again a statutory requrieemtn for TDS made on the last month.