Credit and Debit note effect

West Bengal sec 22 is as follows :

Where any purchaserbeing a registered dealer to whom a credit note or a debit note has been issuedunder section 44, as a consequence of which the input tax credit or input taxrebate availed of by him in any period in respect of which the purchase ofgoods relates, becomes either short or excess, such registered dealer shallcompensate such short or excess by adjusting the amount of input tax credit or input tax rebate allowed to him in respect of the tax period in which thecredit note or debit note, as the case may be, has been issued subject to suchconditions as may be prescribed.

In view of the above , provision suppose if the seller issues the debit note in the month of Jan , 08 and purchaser records the credit note in books of account in the month of Feb ,2008.

In this case , as per my understanding the increase in input tax credit shall be in Jan ,2008 but generally the Vat concept is based on books of accounts. So , practically how is it done?

Please let me know your suggestions/responses.