What is the difference between GSTR-1 and GSTR-3B?

GSTR-1 is the statement of your outward supplies: invoice-wise details of sales, credit/debit notes and exports for the period. It is the basis on which your customers see their GSTR-2A/2B and claim input tax credit.

GSTR-3B is the summary return in which you declare your total tax liability, claim input tax credit and pay the tax. GSTR-1 reports what you sold; GSTR-3B is where the tax is actually paid. The two should reconcile, so a mismatch between them is worth fixing early. Due dates depend on whether you file monthly or under the QRMP scheme, so check the GST portal for your own schedule.