RCM of July paid in August, credit input in july or August

RCM liability of july will be paid in cash to govt in august month.... after that you can claim ITC of that

yup

but my question is shall i take the credit in the month of july or in the month of august 

as RCM of july paid in august. liability shown in july, but GST paid in august

See, here ITC is available once you paid the RCM to the Govt.

If RCM paid to Govt. in 20th Aug 2017 of the outstanding amount of July 2017. the ITC is available in the month of Aug 2017.

can claim the credit of such RCM liability in the same month of return, and you have to show it in both columns i.e. 3.1 (d) and 4 (A) (3) of GSTR-3B.

this not logic ............ bocz supplier is also paying tax in august ........ but client can claim in july month only.