GSTR3B

I have transitional credit from old law so if i want claim that i have to fill trans form1 and file gstr3b before that we have to calculate the tax liability (liabilty after taking the trans credit in to account) and need to pay before 20th aug so my doubt is in aug also my purchase is more than my sales so i don't have to pay anything but to show this shall o need to file any forn before 20th aug or i can go directly file trans 1 and gstr3b before 28th aug? pls clarify