sir in the case of rcm the time of supply as per gst law sec 13(3)
(3) In case of supplies in respect of which tax is paid or liable to be paid on reverse charge basis, the time of supply shall be the earlier of the following dates, namely:––
(a) the date of payment as entered in the books of account of the recipient or the date on which the payment is debited in his bank account, whichever is earlier; or
(b) the date immediately following sixty days from the date of issue of invoice or any other document, by whatever name called, in lieu thereof by the supplier:
Provided that where it is not possible to determine the time of supply under clause (a) or clause (b), the time of supply shall be the date of entry in the books of account of the recipient of supply
so as far as rcm of July 17 / aug 17/ sept 17 is concerned the time of supply has already taken place hence how we can pay the same in current year and take credit as these invoices are already accounted in fy 17-18 rcm was not paid ????