Think of Section 37 as your monthly report card to the GST department — except here, you're reporting everything you sold (outward supplies) to your customers. This is filed through GSTR-1, and it is the backbone of the entire GST matching mechanism.
Who must file, and by when? Every registered person must furnish details of outward supplies electronically on or before the 10th of the following month. So, for sales made in March, GSTR-1 is due by 10th April. But here's the catch the exam loves: there is a blackout window from the 11th to the 15th of the succeeding month — you simply cannot file during this period. Filing reopens from the 16th onwards. The Commissioner can extend this deadline by notification, and any extension by a State/UT Commissioner is automatically treated as extended by the Central Commissioner too.
What counts as 'outward supplies' here? The Explanation at the end of the section clears this up — it includes not just invoices, but also debit notes, credit notes, and revised invoices. So if you issued a credit note to Mr. Sharma in October, that goes into your October GSTR-1.
Accepting/Rejecting auto-populated details (Sub-section 2): Once your supplier files their GSTR-1, those details get auto-populated in your GSTR-2A. If you've been communicated details from Section 38, you must accept or reject them between the 15th and 17th of the following month — not before the 15th, not after the 17th. This is a frequently tested window in MCQs.
Rectification of errors (Sub-section 3): Made a mistake in GSTR-1? You can correct it in a subsequent return. But there's a hard deadline for rectification: the earlier of (a) GSTR-1 for September of the next financial year, or (b) the Annual Return. Miss this window and the error is locked in. If the error caused a short payment of tax, you also pay interest along with the differential tax. This is asked frequently as a 4-mark question — know both the deadline and the interest consequence.
📊 Worked example
Example 1 — Due date and blackout period
Rajesh & Co. Pvt. Ltd. made taxable sales worth ₹12,50,000 in the month of August 2025. The accountant wants to know the due date for GSTR-1 and whether filing on 13th September 2025 is allowed.
Working:
- Tax period = August 2025
- Succeeding month = September 2025
- Due date for filing outward supply details = 10th September 2025
- Blackout window = 11th September to 15th September 2025
- 13th September falls in the blackout window → Filing is NOT permitted
Answer: Rajesh & Co. must file by 10th September. Filing on 13th September is blocked. They can file from 16th September onwards (though the return is already late).
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Example 2 — Rectification deadline
Ms. Iyer runs a trading firm. She filed her GSTR-1 for November 2024 but later discovered she had under-reported sales by ₹2,00,000 (GST @ 18% = ₹36,000 short). She wants to rectify this. The annual return for FY 2024-25 was filed on 28th November 2025. GSTR-1 for September 2025 was filed on 10th October 2025.
Working:
- Rectification deadline = earlier of:
- (a) GSTR-1 for September 2025 (the September following end of FY 2024-25) → filed 10th October 2025
- (b) Annual Return for FY 2024-25 → filed 28th November 2025
- Earlier date = 10th October 2025
- Ms. Iyer must rectify on or before 10th October 2025
- Tax payable on rectification = ₹36,000 + interest under Section 50
Answer: Last date to rectify is 10th October 2025. She must pay ₹36,000 in tax plus applicable interest.
⚠️ Common exam mistakes
- Students write the due date as the 11th — the section says on or before the tenth. The 11th is the first day of the blackout window, not the due date. Always write 10th.
- Forgetting the blackout period entirely — many students say GSTR-1 can be filed anytime after the 10th. Wrong. The 11th–15th window is a hard block. Mention it explicitly in theory answers.
- Confusing the accept/reject window — students write 'by the 17th' without mentioning it cannot be done before the 15th. The window is strictly 15th to 17th. Both limits matter in exams.
- Assuming rectification is always allowed — don't say errors can be corrected 'in the next return' without qualification. The outer limit is September GSTR-1 or Annual Return, whichever is earlier. Ignoring this gets you zero marks on a rectification question.
- Omitting interest when short payment is involved — whenever a rectification covers a tax shortfall, interest under Section 50 is payable. Don't just mention tax — always add 'plus interest' to your answer.
📖 Bare Act text — Section 37, CGST Act 2017
(click to expand)
(1) Every registered person, other than an Input Service Distributor, a non-resident taxable person and a person paying tax under the provisions of section 10 or section 51 or section 52, shall furnish, electronically, in such form and manner as may be prescribed, the details of outward supplies of goods or services or both effected during a tax period on or before the tenth day of the month succeeding the said tax period and such details shall be communicated to the recipient of the said supplies within such time and in such manner as may be prescribed: Provided that the registered person shall not be allowed to furnish the details of outward supplies during the period from the eleventh day to the fifteenth day of the month succeeding the tax period: Provided further that the Commissioner may, for reasons to be recorded in writing, by notification, extend the time limit for furnishing such details for such class of taxable persons as may be specified therein: Provided also that any extension of time limit notified by the Commissioner of State tax or Commissioner of Union territory tax shall be deemed to be notified by the Commissioner.
(2) Every registered person who has been communicated the details under sub-section (3) of section 38 or the details pertaining to inward supplies of Input Service Distributor under sub-section (4) of section 38, shall either accept or reject the details so communicated, on or before the seventeenth day, but not before the fifteenth day, of the month succeeding the tax period and the details furnished by him under sub-section (1) shall stand amended accordingly.
(3) Any registered person, who has furnished the details under sub-section (1) for any tax period and which have remained unmatched under section 42 or section 43, shall, upon discovery of any error or omission therein, rectify such error or omission in such manner as may be prescribed, and shall pay the tax and interest, if any, in case there is a short payment of tax on account of such error or omission, in the return to be furnished for such tax period: Provided that no rectification of error or omission in respect of the details furnished under sub-section (1) shall be allowed after furnishing of the return under section 39 for the month of September following the end of the financial year to which such details pertain, or furnishing of the relevant annual return, whichever is earlier.
Explanation.––For the purposes of this Chapter, the expression ―details of outward supplies‖ shall include details of invoices, debit notes, credit notes and revised invoices issued in relation to outward supplies made during any tax period.
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