GSTR-3B, explained

GSTR-3B is a monthly (or quarterly) summary return. Unlike GSTR-1, which is invoice detail, GSTR-3B is where you total up your tax, claim input credit, and pay the balance.

GSTR-1 vs GSTR-3B

GSTR-1 reports each sale. GSTR-3B nets your output tax against your input tax credit and shows what you owe. The two should reconcile.

When it is due

Usually the 20th of the next month for monthly filers. Late filing attracts interest and a late fee, so the date matters.