TL;DR

September 2026 is an important compliance month for Indian businesses because it includes the second advance-tax instalment deadline on 15 September, regular GST return deadlines, TDS payment obligations and the 30 September tax-audit deadline for AY 2026–27. TDS for August is generally payable by 7 September, while monthly GST taxpayers generally have GSTR-1 and GSTR-3B deadlines on 11 and 20 September respectively. Businesses should reconcile their books, GST records, TDS data and advance-tax calculations before the respective deadlines.

Introduction

September is one of the most important months in the Indian tax-compliance calendar. Businesses, professionals, employers and taxpayers have multiple statutory obligations to track during the month.

From TDS payment and GST return filing to advance tax and tax audit, missing even one deadline can result in interest, late fees, penalties or other compliance issues.

September 2026 is particularly important because taxpayers earning business or professional income need to make the second instalment of advance tax by 15 September, while taxpayers covered by tax-audit requirements for FY 2025–26 / AY 2026–27 need to furnish their tax-audit report by 30 September in the standard cases.

This September 2026 tax compliance calendar provides a practical overview of the major Income Tax, GST and TDS deadlines that businesses and taxpayers should track.

September 2026 Tax Compliance Calendar at a Glance

Date	Compliance	Who Should Check It
7 September 2026	TDS payment for August 2026	Tax deductors
11 September 2026	GSTR-1 for August 2026	Monthly GST filers
13 September 2026	IFF for August 2026	Eligible QRMP taxpayers opting for IFF
15 September 2026	Second advance-tax instalment	Taxpayers liable to pay advance tax
20 September 2026	GSTR-3B for August 2026	Monthly GST filers
30 September 2026	Tax-audit report for AY 2026–27	Taxpayers subject to tax audit

These are the key recurring deadlines relevant to September. Applicability can vary based on taxpayer category, filing frequency, transaction type and any specific statutory extension.

7 September 2026: TDS Payment for August

Tax deductors generally need to deposit TDS deducted during August 2026 by 7 September 2026.

The Income Tax Department confirms that the general TDS payment rule continues to require deposit by the 7th of the following month, with specified exceptions for certain challan-cum-statement transactions and March deductions. The corresponding rules under the Income Tax Act, 2025 retain the same basic timeline.

Businesses should verify:

• TDS deducted during August
• Applicable TDS rates
• PAN details of deductees
• TDS ledger balances
• Challan amounts
• Date of deduction
• Date of payment or credit
• Applicable new Income Tax Act provisions

Why TDS reconciliation matters

Simply depositing TDS is not enough. Businesses should also ensure that the amount deducted in their books agrees with the amount deposited and subsequently reported.

Incorrect TDS can create problems during quarterly statement filing and may result in interest or other consequences.

11 September 2026: GSTR-1 for August

Monthly GST taxpayers generally need to furnish GSTR-1 for August 2026 by 11 September 2026.

GSTR-1 reports outward supplies and therefore plays an important role in the GST compliance chain.

Before filing, businesses should verify:

• B2B invoices
• B2C supplies
• Export transactions
• Credit notes
• Debit notes
• Amendments
• GSTIN of customers
• Invoice numbers
• Taxable value
• GST rates
• E-invoice data, where applicable

Errors in GSTR-1 can subsequently affect recipients’ Input Tax Credit and create reconciliation issues.

13 September 2026: IFF for QRMP Taxpayers

Eligible taxpayers under the QRMP scheme who choose to use the Invoice Furnishing Facility (IFF) should also track the applicable September deadline.

For the August 2026 period, the relevant IFF filing deadline falls on 13 September 2026.

IFF allows eligible QRMP taxpayers to furnish specified invoice details for the first two months of a quarter.

Businesses using IFF should reconcile invoices before uploading them rather than relying on corrections later.

15 September 2026: Second Advance Tax Instalment

One of the most important September deadlines is the second advance-tax instalment.

For taxpayers liable to pay advance tax, the September instalment is generally due on 15 September.

Under the new Income Tax Act, 2025 framework, the prescribed instalment schedule continues to require cumulative payment of 45% of the estimated annual advance-tax liability by 15 September, after reducing amounts already paid in earlier instalments.

Who should pay advance tax?

Advance tax can apply to individuals, businesses, professionals, companies and other taxpayers whose estimated tax liability meets the applicable conditions.

For businesses, advance-tax calculations should consider expected:

• Business income
• Professional income
• Capital gains
• Interest income
• Rental income
• Other taxable income
• TDS/TCS credits
• Previous advance-tax payments

Why September advance tax is important

Businesses often underestimate their tax liability because they calculate advance tax only using previous-year figures.

Instead, taxpayers should review their current-year income and expected tax liability before making the September payment.

A significant shortfall can potentially result in interest consequences under the applicable provisions. Under the Income Tax Act, 2025, Section 424 corresponds to the earlier Section 234B and Section 425 corresponds to Section 234C.

How Businesses Should Calculate September Advance Tax

A practical approach is:

Step 1: Estimate Current-Year Income

Prepare an updated estimate of taxable income for Tax Year 2026–27.

Step 2: Calculate Estimated Tax

Calculate tax based on the applicable provisions and tax regime.

Step 3: Reduce Available Credits

Consider eligible:

• TDS
• TCS
• Advance tax already paid
• Other applicable tax credits

Step 4: Compare With Previous Instalments

Calculate the remaining advance-tax liability after considering payments already made.

Step 5: Pay the Required Instalment

Ensure payment is completed by the applicable deadline.

Businesses should revisit their estimates if profits change substantially during the year rather than blindly following the original estimate.

20 September 2026: GSTR-3B for August

Monthly GST taxpayers generally need to file GSTR-3B for August 2026 by 20 September 2026.

GSTR-3B summarises GST liability and eligible Input Tax Credit and is used to discharge the taxpayer’s GST liability.

Before filing, businesses should reconcile:

• Sales register
• Purchase register
• GSTR-1
• GSTR-2B
• IMS records, where applicable
• Credit and debit notes
• Reverse-charge transactions
• GST payments
• Previous-period adjustments

GSTR-2B vs GSTR-3B reconciliation

Businesses should not assume that every amount appearing in GSTR-2B is automatically eligible for ITC.

ITC eligibility still depends on the applicable legal conditions.

A proper reconciliation should therefore consider both GST portal data and the underlying purchase documents.

30 September 2026: Tax Audit Report

For FY 2025–26 / AY 2026–27, taxpayers covered by tax-audit requirements generally need to furnish their tax-audit report by 30 September 2026.

The Income Tax Department specifically confirms that the tax-audit report for AY 2026–27 is due on 30 September 2026.

For FY 2025–26, the applicable forms continue under the Income-tax Act, 1961:

• Form 3CA, where accounts are audited under another law
• Form 3CB, in other applicable cases
• Form 3CD, containing the prescribed statement of particulars

Important 2026 transition point

Businesses should not confuse FY 2025–26 tax audit requirements with the new Tax Year 2026–27 framework.

For Tax Year 2026–27, the new Income Tax Act, 2025 uses Section 63 for tax audit, corresponding to old Section 44AB. The new Form 26 consolidates the earlier Forms 3CA, 3CB and 3CD, and its tax-audit due date is 30 September 2027.

Therefore:

FY 2025–26 → AY 2026–27 → Old Act → Forms 3CA/3CB/3CD → 30 September 2026

Tax Year 2026–27 → New Act → Section 63 → Form 26 → 30 September 2027

This distinction is extremely important during the 2026 transition.

Other GST Compliance Tasks to Review in September

Apart from statutory return deadlines, businesses should use September to conduct regular GST compliance checks.

Reconcile GSTR-2B

Compare GSTR-2B against:

• Purchase register
• Supplier invoices
• GST portal data
• Credit notes
• Debit notes

Check Vendor Compliance

A supplier’s delayed or incorrect reporting can affect the recipient’s GST reconciliation.

Businesses should identify suppliers whose invoices are missing or incorrectly reported.

Review E-Invoices

Where e-invoicing applies, businesses should verify that eligible invoices have been correctly reported and that IRNs and QR codes are available as required.

Review E-Way Bills

For businesses transporting goods, e-way-bill records should be checked against invoices and actual movement of goods.

TDS Compliance Checklist for September

Businesses should review more than just the 7 September payment deadline.

Check TDS Deduction

Verify whether TDS was deducted on applicable payments such as:

• Professional fees
• Contractor payments
• Rent
• Commission
• Salary
• Interest
• Other applicable payments

Reconcile TDS Ledger

Compare the accounting records with TDS challans.

Verify PAN

Incorrect PAN details can cause reporting and credit-related issues for deductees.

Prepare for Quarterly TDS Reporting

September is also the final month of the July–September TDS quarter. Businesses should maintain accurate records now rather than waiting until the quarterly statement deadline in October.

The Income Tax Department’s current framework provides quarterly TDS statements, with the July–September quarter generally due in October.

September 2026 Business Tax Compliance Checklist

Businesses can use this simple checklist:

Income Tax

• Review advance-tax liability
• Pay the second advance-tax instalment by 15 September
• Review TDS/TCS credits
• Reconcile AIS and tax records
• Prepare tax-audit documentation
• Check tax-audit applicability
• Track the 30 September audit-report deadline

GST

• File GSTR-1 by the applicable deadline
• File GSTR-3B by the applicable deadline
• Reconcile GSTR-2B
• Review ITC eligibility
• Check e-invoices
• Review e-way bills
• Reconcile GST with books

TDS

• Deposit August TDS by 7 September
• Reconcile TDS payable
• Verify PAN details
• Check TDS deductions
• Maintain quarterly TDS records

Accounting

• Reconcile bank accounts
• Review debtors and creditors
• Record missing invoices
• Review expenses
• Check statutory liabilities
• Update books before tax-audit finalisation

Common Tax Compliance Mistakes to Avoid in September

1. Missing the Advance-Tax Deadline

The 15 September deadline is easy to overlook because it falls between monthly GST deadlines.

2. Paying Advance Tax Without Updating Income Estimates

Using an outdated profit estimate can result in an incorrect instalment.

3. Filing GST Returns Without Reconciliation

Filing GSTR-1 and GSTR-3B directly from accounting software without checking portal data can create discrepancies.

4. Ignoring GSTR-2B Differences

Every mismatch should be classified and investigated instead of simply being carried forward.

5. Delaying Tax Audit Preparation

Waiting until the final week of September can make it difficult to resolve accounting and documentation issues.

6. Confusing FY 2025–26 With Tax Year 2026–27

The 2026 transition between the old and new Income Tax Acts makes tax-year identification especially important.

7. Ignoring TDS Reconciliation

A TDS payment may be made on time while errors remain in the underlying deduction or accounting records.

How a CA Can Help Businesses During September

September can become particularly demanding for businesses with GST, TDS, advance-tax and audit obligations.

A Chartered Accountant can assist with:

• Advance-tax calculations
• GST return filing
• GSTR-2B reconciliation
• TDS compliance
• Tax-audit preparation
• Books-of-account review
• AIS reconciliation
• Tax planning
• Compliance-calendar management
• Identifying potential tax risks

For growing businesses, having a structured monthly compliance process can be much more effective than dealing with each deadline separately.

Conclusion

September 2026 is a critical tax-compliance month for Indian businesses and taxpayers.

The key dates to remember are 7 September for August TDS payment, 11 September for monthly GSTR-1, 13 September for applicable IFF filing, 15 September for the second advance-tax instalment, 20 September for monthly GSTR-3B and 30 September for the tax-audit report for AY 2026–27.

Taxpayers should also remember that the transition to the Income Tax Act, 2025 does not mean that all 2026 compliances suddenly move to the new framework. FY 2025–26 / AY 2026–27 continues under the old Act, while income earned from FY 2026–27 is governed by the new Act.

A proactive review of accounting records, GST data, TDS, advance-tax calculations and audit documents can help businesses avoid last-minute compliance problems.

Frequently Asked Questions

1. What is the most important tax deadline in September 2026?

The 15 September 2026 advance-tax deadline and 30 September 2026 tax-audit deadline are among the most important September deadlines. Monthly GST and TDS obligations also need to be tracked.

2. What is the advance-tax due date in September 2026?

The second advance-tax instalment is due on 15 September 2026. The prescribed cumulative payment for this instalment is generally 45% of the estimated annual advance-tax liability, after reducing earlier instalments.

3. What is the TDS payment due date in September 2026?

TDS deducted during August 2026 is generally payable by 7 September 2026, subject to specific exceptions under the applicable rules.

4. What is the GSTR-1 due date in September 2026?

For monthly filers, GSTR-1 for August 2026 is generally due on 11 September 2026.

5. What is the GSTR-3B due date in September 2026?

For monthly filers, GSTR-3B for August 2026 is generally due on 20 September 2026, subject to applicable rules and notifications.

6. What is the tax-audit due date for FY 2025–26?

The tax-audit report for FY 2025–26 / AY 2026–27 is due by 30 September 2026 in the standard cases covered by that deadline.

7. Which tax-audit forms apply for FY 2025–26?

For FY 2025–26, the applicable framework uses Form 3CA or Form 3CB along with Form 3CD, depending on the taxpayer’s circumstances.

8. Does the new Income Tax Act, 2025 apply to FY 2025–26 tax audit?

No. FY 2025–26 / AY 2026–27 tax-audit reporting continues under the Income-tax Act, 1961. The new Income Tax Act, 2025 applies to income of Tax Year 2026–27 onward.

9. Is September advance tax applicable to businesses?

Yes, businesses and other taxpayers who meet the applicable advance-tax conditions need to pay the required instalment by the prescribed deadline.

10. Why should businesses reconcile GST before filing GSTR-3B?

Reconciliation helps identify differences between books, invoices, GSTR-2B, supplier reporting and other GST records, reducing the risk of incorrect ITC claims or tax reporting.

11. What happens if advance tax is not paid correctly?

A shortfall or deferment of advance tax can result in interest under the applicable provisions. The new Income Tax Act, 2025 retains the relevant interest framework, including Sections 424 and 425 corresponding to Sections 234B and 234C of the old Act.

12. Should businesses prepare for tax audit before September?

Yes. Businesses subject to tax audit should ideally start reconciliation and documentation well before the deadline so that accounting, GST, TDS and other discrepancies can be resolved before the audit report is filed.

 

Blog By : CA Junaid Khan

Written by CA Junaid Ahmed Khan, Junaid Khan & Co., Bhopal.