Please sir explain 180 days

Please clarify why you calculate 180 days ? (question not clear)

As a GST Registered dealer, you would have availed ITC on inward supplies. But, if you fail to pay the invoice amount to the supplier (i.e Sundry Creditors) within 180 days from the date of invoice, the ITC earlier taken by you at time of purchase has to be reversed. If part of the invoice is paid the ITC will be reversed on a proportionate basis.
After that when you paid to Creditors, than you can reclaim your reversed credit.

E.g.Mr. XYZ received goods on 1st July 2017 worth ₹ 10000 on which GST ₹ 1800 was charged.

Mr. XYZ claimed the GST of ₹ 1800 as ITC in his GSTR 3B

Mr. XYZ could not pay the invoice amount until December 2017.

This means that Mr. XYZ will have to reverse the ITC of ₹ 1800 while filing GSTR – 3B for December 2017 in January 2018.

Thank you so much sagar sir

Most welcome...