GST filing used to forgive you. Get a number wrong, adjust it in the summary return, move on. That system is gone: your GSTR-3B is now locked to your GSTR-1, the Invoice Management System decides what credit you get, and a return more than three years overdue can no longer be filed at all.
Most regular taxpayers file GSTR-1 by the 11th and GSTR-3B by the 20th of the following month, and since July 2025 the outward-liability figures in GSTR-3B are auto-locked from GSTR-1 — the only correction route is GSTR-1A, filed before GSTR-3B.
The bottom line
The two returns that matter: GSTR-1 for outward sales by the 11th, GSTR-3B for the summary and the payment by the 20th. QRMP filers are staggered to the 22nd or 24th.
What got strict: GSTR-3B's outward figures are locked from GSTR-1, and the Invoice Management System is effectively mandatory from April 2026 and now gates your input tax credit.
The hard stop: from January 2026, any return more than three years past its due date is permanently blocked.
The returns you actually file
- GSTR-1 — outward supplies, invoice by invoice. This data flows to your buyers.
- GSTR-3B — the summary return where you declare total sales, claim input tax credit and pay the tax.
- GSTR-2B — auto-generated on the 14th of each month, showing your eligible ITC. You do not file it.
- GSTR-9 and 9C — the annual return and reconciliation statement, for taxpayers they apply to.
- CMP-08 and GSTR-4 — for composition scheme taxpayers.
Businesses with turnover up to ₹5 crore can opt into the QRMP scheme: quarterly returns with monthly tax payment, using the Invoice Furnishing Facility to upload invoices each month.
Deadlines
| Return | Who / frequency | Typical due date |
|---|---|---|
| GSTR-1 | Monthly filers | 11th of the next month |
| GSTR-1 (IFF) | QRMP, optional monthly upload | 13th of the next month |
| GSTR-3B | Monthly filers | 20th of the next month |
| GSTR-3B | QRMP (quarterly) | 22nd or 24th, by state group |
| GSTR-9 | Annual (if applicable) | 31 December of next FY |
Dates move by notification more often than you would like — the March 2026 GSTR-3B was extended to 21 April 2026, for instance — so confirm on the GST portal rather than from a table anyone published earlier.
The hard-locking of GSTR-3B
From the July 2025 tax period, the outward-liability tables in GSTR-3B, Tables 3.1 and 3.2, are auto-populated from your GSTR-1 or IFF and locked. You cannot edit those figures on the portal, and there is no override.
Which means a wrong GSTR-1 produces a wrong GSTR-3B, and the only way out is through the GSTR-1 data. That one change is why GSTR-1 accuracy stopped being a bookkeeping preference and became the thing the whole month rests on.
ITC hard-locking in Table 4 is expected to follow, in a second phase around mid-2026, which will restrict your credit to exactly what comes through GSTR-2B. The reconciliation habit you build now is the one you will need then.
GSTR-1A, the correction route
To fix an error in a filed GSTR-1 for the same tax period, use GSTR-1A. It is an amendment form filed after GSTR-1 and before GSTR-3B for that period, and it handles a wrong GSTIN, taxable value or tax amount, or a response to a buyer's rejection.
Two constraints make it unforgiving. GSTR-1A can be filed only once per period, and it cannot itself be revised. So gather every correction before you open it.
The Invoice Management System
IMS is the new gatekeeper for input tax credit. Every invoice your supplier uploads arrives in your IMS dashboard, where you accept, reject or keep it pending. Only accepted invoices flow into your GSTR-2B and become claimable ITC.
Here is the rule that catches people: inaction equals acceptance. Do nothing before GSTR-2B generates and the invoice is deemed accepted, entering your credit with whatever errors it carries.
IMS became fully operational on 1 October 2025 and is effectively mandatory from April 2026, with the portal blocking ITC on invoices that are not reflected in GSTR-2B. A weekly IMS review, before the 14th, is the routine this asks for.
The three-year lock
From 1 January 2026, GST returns — GSTR-1, GSTR-3B, GSTR-4 and GSTR-5 among them — cannot be filed once three years have passed from their original due date.
Past that line the period is permanently blocked. Unreported tax and unclaimed ITC for it are both lost, while your exposure to demand and penalty carries on. If you have old pending returns sitting in a drawer, the deadline for doing something about them is real and it does not move.
Late fees and interest
- Late fee: broadly ₹50 per day, being ₹25 CGST and ₹25 SGST, for returns carrying tax, and ₹20 per day for nil returns, subject to caps.
- Interest: 18% per annum on tax paid late, under Section 50.
- The indirect cost: a late GSTR-1 delays your buyers' ITC, which is a commercial problem before it is a compliance one.
One more thing to check on every invoice. Under GST 2.0, from 22 September 2025 the rate structure was rationalised towards mainly 5% and 18% slabs, with the 12% and 28% slabs largely removed. Billing at an outdated rate produces a non-compliant invoice.
A worked example
A trader files GSTR-1 by the 11th. A supplier mistakenly uploads an invoice to the trader's GSTIN for ₹1,80,000 instead of ₹18,000.
Because inaction counts as acceptance in IMS, if the trader does not reject it before GSTR-2B generates on the 14th, the inflated credit flows straight into a GSTR-3B that cannot be edited. That is ITC the trader is not entitled to, and it comes back later as a reversal with 18% interest attached.
The fix takes minutes and only works if it happens in time: review IMS weekly, reject the wrong invoice, and let only correct invoices feed the return.
Common mistakes
- Leaving the IMS dashboard unreviewed. Unactioned invoices are deemed accepted into your ITC.
- Treating GSTR-1 as a rough draft. Its errors now flow into a locked GSTR-3B.
- Trying to adjust GSTR-3B manually. The outward-liability fields do not accept edits; fix GSTR-1 through GSTR-1A.
- Leaving old returns pending. The three-year lock eventually makes them unfilable.
- Billing at pre-GST 2.0 rates.
Frequently asked questions
What are the due dates for GSTR-1 and GSTR-3B? For monthly filers, GSTR-1 by the 11th and GSTR-3B by the 20th of the following month. QRMP filers have staggered quarterly dates.
Can I edit GSTR-3B if my GSTR-1 had an error? No. The outward-liability fields are hard-locked from GSTR-1. Correct the data through GSTR-1A before filing GSTR-3B.
What is IMS in GST? The Invoice Management System, where you accept, reject or pend supplier invoices. Only accepted invoices give you ITC, and taking no action counts as acceptance.
What happens to GST returns older than three years? From January 2026 they are permanently blocked and can no longer be filed.
What is the interest on late GST payment? 18% per annum on the tax paid late, under Section 50, plus the applicable late fees.
Do I have to review IMS if I have very few suppliers? Yes. The deemed-acceptance rule does not scale with volume — one wrong invoice nobody rejected is enough.