Rcm interest

If any liability of RCM of March, 19 i.e. FY 18-19 is left to be disclosed in the Return of March, 19 then the same can be shown last, by the due date of filing of GSTR3B of September, 19. If the same has not been shown in the return of September, 19 then the last option is to disclose it in Annual Return of FY 18-19.
You cannot pay it in March, 2020. And Interest would be applicable @ 18% for the period between the due date of filing of March,19 Return and the date of filing Annual Return.

Further, you will now not be eligible to take ITC on that RCM, as the due date to do the same was of September, 19 only.

Interest needs to be paid. It cannot be avoided. Interest @ 18% p.a. needs to be paid along with your RCM Tax Liability. Also as said by Himanshi Ma'am, you have to disclose it in FORM GSTR-9 of FY 2018-19 and dispose the tax via Challan DRC 03.

Kindly note that GST is a "SELF ASSESSED TAX". Section 59 of the CGST Act 2017 clearly tells that GST is a self assessed tax. So it is your responsibility to calculate and pay Interest now.

I agree, but i want to know about interest period whether it will freeze till payment of tax date only or it will continue to be levied till interest payment also.

Interest will be levied till filing of DRC03.
I would suggest you to deposit the challan against RCM along with Interest and raise liability via DRC03.

Further, at the time of filing of GSTR9 for the FY 18-19 just declare the figures of RCM there. You will not be required to deposit that amount again as you would have already deposited the same. If you will wait to deposit challan at the time of filing of GSTR9 then it will lead to higher amount of Interest because no one knows when will our Annual Return of FY 18-19 be get filed.

So, the better option is to deposit the challan of tax plus interest now and set off your liability with DRC03. Don't declare this in any GST3B. Just disclose this fact in you GSTR9 of 18-19 whenever you file it.

Thanks