September 2026 GST, TDS & Advance Tax Calendar: What You Need to Know for Simple Filing

C P C Services — Thu, 03 Sep 2026 14:47:13 +0000

Every deadline your business needs to hit in September 2026 — explained simply, with what each filing involves and exactly what it costs to miss it.

September is the most compliance-intensive month of the financial year for Indian businesses. Three major obligations converge in a single 30-day window: GST return filings for August, the second installment of advance tax for FY 2026-27, and the monthly TDS deposit cycle. Most crucial is furnishing of Audit Report under section 44AB (of Income-tax Act, 1961 as applicable for FY 2025-2026) on or before due date i.e. 30th September 2026. A single day delay may cost a heavy penalty which may extend to 1.50 Lakh.

For businesses with turnover above ₹5 crore, GSTR-3B is due on 20 September. The advance tax second instalment — 45% of estimated annual liability, cumulative — is due on 15 September. The TDS deposit for August deductions is due on 7 September. If your business falls under all three, you have a significant cash flow obligation in a single fortnight.

This blog gives you every deadline for September 2026, explains what each involves, and gives you the penalty calculation for any you miss — so you can prioritize correctly and avoid paying a rupee more than you owe.

KEY TAKEAWAY

September 2026 is critical for two key reasons. 15 September is the second advance tax instalment, requiring cumulative payment of 45% of estimated annual tax, with interest implications for shortfalls. The FY 2025–26 ITC final window also closes this month, so eligible unclaimed ITC must be claimed in the September GSTR-3B before the deadline.

Finally, Q2 closes on 30 September. QRMP taxpayers will file their quarterly GSTR-3B in October, but businesses should complete their books reconciliation, ITC matching, and supplier follow-ups before 30 September to stay prepared.

All September 2026 Compliance Deadlines at a Glance

This is your complete compliance calendar for September 2026. Dates shaded in blue are GST deadlines; those in green are income tax and TDS deadlines.

Type	Due Date	Form / Filing	Who It Applies To	Cost of Missing It
INCOME TAX & TDS	7 Sep 2026	TDS Deposit	All non-government deductors: deposit TDS deducted in August 2026	1.5%/month interest from deduction date + 30% expense disallowance risk
INCOME TAX & TDS	7 Sep 2026	TCS Deposit	E-commerce operators and notified TCS collectors: deposit TCS for August 2026	Same interest and disallowance as TDS
INCOME TAX & TDS	15 Sep 2026	Advance Tax Q2	All taxpayers with estimated tax liability above ₹10,000 after TDS: pay 45% of annual tax (cumulative)	1% per month under Section 234C on shortfall from both June and September instalments
INCOME TAX & TDS	30 Sep 2026	Tax Audit Report	Businesses and professionals requiring tax audit: CA must sign and upload Form 3CD on portal	ITR deadline of 31 October 2026 depends on audit report by 30 September — delays cascade
GST	10 Sep 2026	GSTR-7	Government bodies and notified entities deducting TDS under GST: monthly return for August 2026	₹50/day late fee + blocks deductee’s GST TDS credit
GST	10 Sep 2026	GSTR-8	E-commerce operators collecting TCS under GST: monthly return for August 2026	₹50/day late fee + deductee cannot claim TCS credit
GST	11 Sep 2026	GSTR-1	Monthly filers (turnover >₹5 Cr or opted monthly): outward supply return for August 2026	₹50/day late fee; buyer ITC blocked; hard-locked GSTR-3B cannot compensate
GST	13 Sep 2026	GSTR-5	Non-resident taxable persons: GST return for August 2026	₹50/day late fee; registration may be cancelled for repeated defaults
GST	13 Sep 2026	GSTR-6	Input Service Distributors: ISD return for August 2026	₹50/day late fee; ITC distribution to branches delayed
GST	20 Sep 2026	GSTR-3B	Monthly filers (turnover >₹5 Cr): summary return + tax payment for August 2026	₹50/day late fee + 18% p.a. interest on unpaid tax from 20 Sep
GST	25 Sep 2026	PMT-06	QRMP taxpayers: monthly GST tax payment for August 2026	18% p.a. interest on late payment; quarterly GSTR-3B filing will reflect deficit
GST	30 Sep 2026	GSTR-2B Reconciliation	All registered taxpayers: deadline to complete FY 2025-26 ITC reconciliation before Q2 close	FY 2025-26 ITC lapses after October 2026 GSTR-3B due date — credit permanently lost

QUICK TIP

Not sure if advance tax applies to you? It does if your estimated income tax liability for FY 2026-27 exceeds ₹10,000 after accounting for TDS already deducted from salary, bank interest, contractor payments, etc. If you are salaried and your employer deducts full TDS, you may still owe advance tax if you have rental income, capital gains, FD interest, or any income not covered by employer TDS.

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TDS Compliance in September 2026

TDS Deposit — 7 September 2026

Any TDS deducted from payments made in August 2026 must be deposited with the government by 7 September 2026. This applies to salary TDS (deducted monthly from employee salaries), TDS on contractor payments, professional fees, rent, interest, commission, and any other payment where TDS is applicable under the Income Tax Act, 2025.

Governing law: Since all August 2026 payments occur after 1 April 2026, TDS is now governed by Section 393 of the Income Tax Act, 2025 (not Sections 194C, 194J, 194H etc. of the old Act). When depositing TDS, ensure your challan quotes the correct new section reference. Portal validation may fail if old Act section numbers are used for post-March 2026 transactions.

IMPORTANT WARNING

Two separate penalty streams apply for TDS default:

Interest: 1.5% per month from date of deduction to date of deposit. Part of a month counts as a full month. A deposit made on 20 September instead of 7 September = 1 full month’s interest (not 13 days).

Expense disallowance: 30% of the amount paid to any resident (on which TDS was deductible but not deposited by ITR filing deadline) will be disallowed as a business expense under Section 35(b) of the Income Tax Act, 2025. This can significantly increase your taxable income.

Advance Tax: The 15 September Deadline Explained

The 15 September deadline is the second of four advance tax installments for FY 2026-27. It is also the most misunderstood — because the percentage required is cumulative, not just for the quarter.

Instalment	Due Date	Cumulative % Required	If Missed
1st Instalment	15 June 2026	At least 15% of annual tax	1% per month interest under Section 234C on shortfall
2nd Instalment	15 September 2026	At least 45% of annual tax (cumulative)	1% per month on cumulative shortfall from June + September
3rd Instalment	15 December 2026	At least 75% of annual tax (cumulative)	1% per month on cumulative shortfall from all three
4th Instalment	15 March 2027	100% of annual tax (cumulative)	1% per month under Section 234B on total shortfall if <90% paid

What ‘cumulative 45%’ means in practice: If your estimated annual tax liability is ₹3 lakh, you must have paid a total of ₹1,35,000 by 15 September — including whatever you paid on 15 June. If you paid ₹30,000 in June (10% — below the required 15%), you need to pay at least ₹1,05,000 in September to reach the 45% cumulative threshold. The shortfall from June also attracts interest.

Who is exempt from advance tax:

Senior citizens (above 60 years) with no business or professional income — fully exempt from advance tax
Presumptive taxation taxpayers under Section 44AD and 44ADA (small businesses and professionals with turnover/receipts below the prescribed limits) — can pay 100% of advance tax as a single instalment by 15 March 2027
Salaried individuals whose entire tax liability is covered by employer TDS — no advance tax required

CPC INSIGHT

The most common advance tax error CPC Services corrects for SME clients is underestimation. Business owners estimate their Q2 advance tax based on Q1 actual — without factoring in a large order they have just won, a property sale completed in August, or capital gains from a mutual fund redemption. If your income in Q2 was materially higher than Q1, revise your advance tax estimate before 15 September — overpaying is free; underpaying costs 1% per month.

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GST Filing Deadlines in September 2026

GSTR-1: Outward Supply Return — 11 September 2026

Monthly GST filers must report all B2B and B2C invoices issued in August 2026 by 11 September. With GSTR-3B hard-locked since July 2025, GSTR-1 accuracy is more important than ever — errors cannot be corrected directly in GSTR-3B and must go through GSTR-1A before GSTR-3B is filed.

Critical for September specifically: Any FY 2025-26 invoice that has not yet been reported or was reported with errors must be amended in September’s GSTR-1 (Table 9A). The November 2025 amendment cut-off has passed — September is the last practical window to capture FY 2025-26 output corrections before year-end.
QRMP filers: September is the last month of Q2 (July–September). Your quarterly GSTR-1 for Q2 is not due until 13 October 2026. However, if you have B2B buyers whose ITC depends on seeing your August invoices in their September GSTR-2B, use the Invoice Furnishing Facility (IFF) before 13 September to upload those invoices for the month.

GSTR-3B: Summary Return + Tax Payment — 20 September 2026

GSTR-3B for August 2026 is due 20 September for monthly filers. This is simultaneously the most important deadline this month and the one with the most cash flow impact — particularly for businesses that also owe advance tax by 15 September.

September-specific cash flow alert: If you owe both advance tax (15 Sep) and GSTR-3B tax payment (20 Sep), plan your liquidity carefully. Both are cash outflows within 5 days of each other. For a business with ₹50 lakh monthly GST turnover, the GSTR-3B net tax payment alone can be ₹3–8 lakh. Add the advance tax instalment and the combined cash requirement in one fortnight is material.

REMINDER

September GSTR-3B is the final return for FY 2025-26 ITC claims. Any input tax credit from FY 2025-26 (April 2025–March 2026) that has not yet been claimed must be included in this return. After the 20 October 2026 due date for September GSTR-3B, unclaimed FY 2025-26 ITC lapses permanently. There are no extensions for this deadline.

PMT-06: Monthly GST Payment for QRMP Taxpayers — 25 September 2026

QRMP taxpayers file GST returns quarterly but pay tax monthly. The August 2026 tax payment via Form PMT-06 is due by 25 September. The amount can be calculated using either the Fixed Sum Method (35% of previous quarter’s GSTR-3B tax) or the Self-Assessment Method (based on actual August ITC and liability). If August was a high-revenue month compared to last quarter, the Fixed Sum Method may underpay — use the Self-Assessment Method for accuracy.

Tax Audit Report Deadline — 30 September 2026

Businesses and professionals whose books are required to be audited under Section 44AB must have their Chartered Accountant sign and upload the tax audit report (Form 3CD/3CA/3CB) by 30 September 2026. The ITR filing deadline for audit cases is 31 October 2026 — which is directly dependent on the audit report being completed first.

Who this applies to:

Businesses with turnover above ₹1 crore (₹10 crore if cash transactions are below 5%), and professionals with gross receipts above ₹50 lakh (₹75 lakh under presumptive scheme). Directors of private limited companies and partners of auditable firms also need to ensure the entity-level audit is complete before their personal ITR deadline.

ADVISORY

Audit and ITR are separate obligations but one blocks the other. If your CA has not completed the tax audit report by 30 September, your ITR cannot be filed by 31 October. A late ITR for audit cases attracts ₹5,000 late fee and more significantly — blocks the ability to choose the old tax regime (the new regime becomes the default for belated returns). Start the audit preparation now, not in the last week.

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What Each Missed Deadline Costs You: September Edition

September’s deadlines are more expensive to miss than most months — because three of them carry compounding consequences.

Deadline	Late Fee	Interest	Hidden Cost
TDS Deposit (7 Sep)	None on deposit	1.5%/month from deduction date	30% expense disallowance if not deposited by ITR date
Advance Tax (15 Sep)	None	1% per month on shortfall under Section 234C	Shortfall from June instalment also starts attracting interest — compounding from both quarters
GSTR-1 (11 Sep)	₹50/day, no cap stated	None on GSTR-1 directly	Buyer’s GSTR-2B blocked → their ITC impacted → they chase you → relationship risk
GSTR-3B (20 Sep)	₹50/day, capped ₹5,000	18% p.a. on unpaid tax from 20 Sep	Final FY 2025-26 ITC lapse — unclaimed credit permanently lost
PMT-06 (25 Sep)	None on payment	18% p.a. from 25 Sep on deficit	Quarterly GSTR-3B will show underpayment and attract further interest
Tax Audit (30 Sep)	0.5% of the turnover or Rs.1.50 Lakh which-ever is lower.	N/A	Failed to Furnish the report, Likely to miss ITR deadline → ₹5,000 late fee + old regime option lost + reputation risk with lenders

Planning Your September Cash Flow: The Fortnight That Matters

For any SME owner juggling GST, TDS, and advance tax simultaneously, September requires deliberate cash flow planning. Here is the sequence of outflows and what to prepare:

Date	Obligation	Cash Flow Planning Action
7 Sep	TDS deposit for August 2026	Calculate total TDS deducted in August across all categories. Ensure bank balance covers this before the 7th. Late TDS deposit is one of the most common inadvertent defaults.
11 Sep	GSTR-1 for August 2026	No cash outflow — but file accurately. Errors here cannot be corrected in GSTR-3B. Review all B2B invoices, credit notes, and advances before submission.
15 Sep	Advance tax Q2 instalment (45% cumulative)	Calculate estimated annual tax, subtract June payment and TDS credits. Pay the balance due to reach 45% cumulative. Use Challan 280 online — select Tax Year 2026-27 and Minor Head 100 (Advance Tax).
20 Sep	GSTR-3B for August 2026 — final FY 2025-26 ITC date	Reconcile with GSTR-2B before filing. Claim all remaining FY 2025-26 ITC in this return. After 20 Oct deadline, any unclaimed credit is lost. Pay net GST due before filing.
25 Sep	PMT-06 for August 2026 (QRMP taxpayers only)	Calculate using Self-Assessment Method if August sales were significantly higher than last quarter. Fixed Sum Method may underpay.
30 Sep	Tax audit report due; Q2 books reconciliation close	Coordinate with your CA for timely audit completion. Finalise Q2 GST-books reconciliation. Last day for QRMP filers to resolve pending FY 2025-26 ITC issues before Q2 closes.