GSTR-1 due dates — every month and every quarter.

1. What GSTR-1 is

GSTR-1 is the return of outward supplies — every sales invoice, credit note and debit note issued during the period. Unlike GSTR-3B, which is a summary return, GSTR-1 is invoice-level: each B2B invoice flows from your GSTR-1 into the buyer's GSTR-2B, where it becomes the basis for the buyer's ITC claim.

Filing GSTR-1 on time is therefore not just your own compliance — it is your customer's ability to claim ITC for the month.

2. Monthly vs Quarterly (QRMP)

Two filing cadences are available:

• Monthly: Mandatory for taxpayers with aggregate turnover above ₹5 crore in the previous FY. Also the default for anyone who has not opted into QRMP.
• Quarterly (QRMP): Available to taxpayers with aggregate turnover up to ₹5 crore in the previous FY. GSTR-1 is filed once per quarter; tax is still paid monthly via GSTR-3B challan (PMT-06).

The QRMP option is exercised quarter-by-quarter on the GST portal — you can switch between monthly and quarterly without penalty, as long as you do it before the start of the quarter.

3. Due date table — monthly filers, FY 2026-27

CBIC notifications occasionally extend a given month's due date (typhoons, portal outages, year-end pressure). Always check the latest CBIC notification before assuming the extension applies to your state.

QRMP — quarterly GSTR-1, FY 2026-27

4. IFF — Invoice Furnishing Facility

QRMP filers can optionally upload B2B invoices monthly via IFF so their buyers see ITC in their next 2B without waiting for the quarter to end. IFF is for Months 1 and 2 of each quarter; Month 3 invoices go straight into the quarterly GSTR-1.

• Cap: ₹50 lakh of invoice value per month
• Due date: 13th of the following month
• Optional: Skipping IFF is not a default; invoices then go into the quarterly GSTR-1

5. GSTR-1A — the amendment return

Introduced from FY 2024-25, GSTR-1A is an optional return filed between GSTR-1 and GSTR-3B of the same period. Use it if you discover a missed invoice or a wrong figure after filing GSTR-1 but before filing 3B — the correction flows into 3B liability immediately, instead of waiting for next month's amendment table.

For FY 2026-27, GSTR-1A is available each month or quarter from the date GSTR-1 is filed until the day GSTR-3B is filed. There is no separate due date — it is bounded by 1 on the left and 3B on the right.

6. Sequential filing rule — Section 39(10)

You cannot file GSTR-3B for a period unless GSTR-1 for the same period is already filed. The portal enforces this. The practical consequence: a late GSTR-1 forces GSTR-3B to be late too — and that is the leg that carries interest.

File GSTR-1 first, even if you have to file a nil or partial return. You can always amend in GSTR-1A or next month's amendment table. What you cannot do is unlock 3B without a 1 on file.

7. Late fees

Late fees accrue per day from the day after the due date until the day the return is filed.

8. Interest

GSTR-1 itself does not carry a tax-payment leg, so it does not directly trigger interest. However, because of the sequential-filing rule, a late GSTR-1 will delay GSTR-3B — and any net tax payable in 3B carries interest at 18% per annum from the original due date of 3B until the date the tax is actually paid.

If a GSTR-1 vs 3B mismatch later triggers a DRC-01B notice and you accept the demand, interest at 18% applies on the tax differential as well.

9. Common errors that trigger DRC-01B

DRC-01B is the auto-generated mismatch notice when GSTR-1 outward supplies exceed GSTR-3B outward supplies beyond the tolerance. The most common causes:

• An invoice was amended in GSTR-1 but the corresponding adjustment was missed in GSTR-3B.
• A credit note was filed in GSTR-1 but not deducted from 3B outward supplies.
• RCM-applicable supplies were included in 1 but excluded from 3B outward (they belong in 3B inward RCM, not outward).
• Export-with-IGST supplies were included in 1 Table 6A but missed in 3B Table 3.1(b).
• Advances received were declared in 1 but the corresponding invoice adjustment in 3B was not made.

A return-prep tool that reconciles GSTR-1 against GSTR-3B before filing catches these before the portal does.

TatvaBooks prepares both GSTR-1 and 3B from your books and reconciles them against your 2B on a single screen, so a 1-vs-3B mismatch surfaces while you can still fix it — before you file on the portal, not after a DRC-01B notice.

10. Frequently asked questions