TL;DR

September 2026 is an important compliance month for Indian taxpayers and businesses. Key deadlines include the 15 September 2026 advance-tax instalment, monthly TDS payment by 7 September, GSTR-1 by 11 September and GSTR-3B by 20 September for monthly filers. The month also includes the 30 September 2026 tax-audit report deadline for applicable FY 2025–26 cases.

For businesses and professionals, September is particularly important because tax audit documentation, GST reconciliations, TDS records and advance-tax calculations often overlap.

Since Tax Year 2026–27 is governed by the Income Tax Act, 2025, taxpayers should also understand the transition between the old and new income-tax provisions. However, the tax audit report for FY 2025–26 / AY 2026–27 continues to be governed by the Income Tax Act, 1961 and uses the existing Forms 3CA/3CB/3CD.

Introduction

September is one of the most important months in the Indian tax compliance calendar.

While many taxpayers associate September mainly with Advance Tax, businesses have several other obligations to track during the month. These can include GST returns, TDS payments, TDS statements, tax-audit preparation and, for eligible taxpayers, the filing of audit reports.

Missing one deadline may result in late fees, interest, loss of certain tax benefits or unnecessary compliance notices.

This makes a September 2026 Tax Compliance Calendar particularly useful for business owners, professionals, freelancers, companies and finance teams.

The important dates should not be considered in isolation. A taxpayer may have multiple obligations falling within the same month, and each one may require different documents and calculations.

September 2026 Tax Compliance Calendar at a Glance

The major September deadlines can be broadly grouped as follows:

7 September 2026 – TDS Payment

TDS deducted during August 2026 is generally required to be deposited with the Government by 7 September 2026, subject to applicable exceptions.

11 September 2026 – GSTR-1

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

15 September 2026 – Second Advance Tax Instalment

Eligible taxpayers generally need to achieve 45% cumulative payment of estimated annual advance tax by 15 September 2026.

20 September 2026 – GSTR-3B

Monthly GST filers generally need to file GSTR-3B for August 2026 by 20 September 2026 and discharge the corresponding GST liability.

30 September 2026 – Tax Audit Report

For applicable FY 2025–26 / AY 2026–27 tax-audit cases, the tax-audit report is due by 30 September 2026.

The exact compliance obligation can vary depending on the taxpayer’s category, filing frequency, audit status and applicable notifications.

7 September 2026: TDS Payment Due Date

One of the first important September compliance dates is the TDS payment deadline.

Generally, tax deducted at source during August 2026 must be deposited with the Central Government by 7 September 2026.

The Income Tax Department confirms that the general rule continues to require TDS to be deposited by the 7th of the following month. The Income Tax Rules, 2026 retain the same general timeline.

For example, if a business deducts TDS on eligible payments made during August, the applicable TDS amount generally needs to be deposited by 7 September.

Why TDS Payment Should Be Checked Carefully

Businesses should reconcile:

• TDS deducted during August
• TDS payable
• TDS actually deposited
• Challan details
• Deductee information
• PAN details
• Applicable TDS provisions

A mismatch between books, challans and TDS statements can create unnecessary compliance problems later.

Are There Exceptions to the 7 September TDS Deadline?

Yes.

Certain types of TDS have separate timelines.

The Income Tax Department specifically notes that certain challan-cum-statement provisions, including specified transactions involving property, rent, payments by certain individuals/HUFs and virtual digital assets under the old provisions, have a different 30-day timeline from the end of the month of deduction.

Therefore, businesses should not assume that every TDS payment follows exactly the same deadline.

11 September 2026: GSTR-1 Due Date

For regular taxpayers filing GST returns monthly, GSTR-1 for August 2026 is generally due on 11 September 2026.

GSTR-1 contains details of outward supplies and is an important source of information for the recipient’s GST records and input-tax-credit reconciliation.

The GST Portal states that the normal due date for monthly GSTR-1 is the 11th day of the succeeding month, unless the Government extends the deadline through notification.

What Should Businesses Check Before Filing GSTR-1?

Before filing, businesses should reconcile their sales data with:

• Sales register
• Tax invoices
• Debit notes
• Credit notes
• E-invoices, where applicable
• E-way bill information
• GST books
• Customer GSTINs

Correct reporting is important because errors in outward-supply data can affect the recipient’s ITC reconciliation.

What Happens If GSTR-1 Is Filed Late?

Late filing can result in applicable late fees and can create downstream compliance problems.

The GST Portal also indicates that failure to file GSTR-1 can trigger a system-generated return-defaulter notice in the prescribed circumstances.

Therefore, businesses should avoid treating GSTR-1 as merely an administrative formality.

15 September 2026: Advance Tax 45% Rule

For many taxpayers, 15 September 2026 is the most important tax deadline of the month.

The second advance-tax instalment for Tax Year 2026–27 is due on this date.

By 15 September, eligible taxpayers generally need to have paid 45% of their estimated annual advance-tax liability cumulatively.

The Income Tax Department’s guidance states the following advance-tax schedule:

• 15 June – 15%
• 15 September – 45%
• 15 December – 75%
• 15 March – 100%

How Does the 45% Rule Work?

The 45% requirement is cumulative.

Suppose your estimated annual advance-tax liability is ₹2,00,000.

By 15 September:

₹2,00,000 × 45% = ₹90,000

If you already paid ₹30,000 in June, you would generally need another:

₹90,000 − ₹30,000 = ₹60,000

Therefore, your cumulative advance-tax payment becomes ₹90,000.

Who Needs to Pay Advance Tax?

Advance tax can apply where the taxpayer’s estimated tax liability reaches the prescribed threshold after considering relevant tax credits.

It can affect:

• Business owners
• Professionals
• Freelancers
• Consultants
• Investors
• Landlords
• Salaried taxpayers with substantial additional income

Income such as interest, rental income, bonds and capital gains can increase tax liability even when salary TDS is already being deducted.

Advance Tax Under the Income Tax Act, 2025

Tax Year 2026–27 is the first tax year governed by the Income Tax Act, 2025.

The Income Tax Department confirms that advance-tax obligations for income earned during FY 2026–27 arise under the new Act. It also states that the policy and interest rates for advance-tax defaults remain unchanged.

Taxpayers should therefore understand both the new section numbers and the corresponding old provisions when researching advance-tax compliance.

20 September 2026: GSTR-3B Due Date

For monthly GST filers, GSTR-3B for August 2026 is generally due on 20 September 2026.

GSTR-3B is a summary GST return through which taxpayers declare their GST liability and discharge the corresponding tax.

The GST Portal confirms that monthly filers generally file GSTR-3B by the 20th day of the succeeding month.

What Should Be Reconciled Before GSTR-3B?

Before filing GSTR-3B, businesses should review:

Output GST

Verify taxable outward supplies and the corresponding CGST, SGST/UTGST and IGST liabilities.

Input Tax Credit

Reconcile eligible ITC with purchase records and available GST information.

Reverse Charge

Check whether any reverse-charge liability applies.

Previous-Period Adjustments

Review amendments, credit notes, debit notes and previous-period corrections.

Tax Payment

Ensure that the GST liability is properly discharged while filing the return.

Why GSTR-1 and GSTR-3B Should Be Reconciled

A common compliance mistake is preparing GSTR-1 and GSTR-3B independently.

The outward-supply figures reported in GSTR-1 should be reconciled with the figures used for GSTR-3B.

Differences can create questions during future GST scrutiny or reconciliation.

30 September 2026: Tax Audit Report Due Date

For taxpayers whose FY 2025–26 accounts are subject to tax audit, 30 September 2026 is a major deadline.

The Income Tax Department specifically confirms that the tax-audit report for FY 2025–26 / AY 2026–27 is due on 30 September 2026 where the corresponding ITR due date is 31 October 2026.

Which Taxpayers May Need a Tax Audit?

Under the Income Tax Act, 2025, Section 63 corresponds to old Section 44AB.

The tax-audit thresholds remain broadly unchanged:

For business, the threshold is generally ₹1 crore, increasing to ₹10 crore where cash receipts and cash payments do not exceed the prescribed 5% conditions.

For professionals, the gross-receipts threshold is generally ₹50 lakh, subject to the applicable provisions.

Other circumstances can also trigger audit requirements, including certain cases involving presumptive taxation.

Which Forms Apply to FY 2025–26?

This is an important transition-year point.

The tax audit report for FY 2025–26 / AY 2026–27 continues to use the existing forms under the Income Tax Act, 1961:

• Form 3CA
• Form 3CB
• Form 3CD

The Income Tax Department has specifically clarified that these forms continue to apply even though the report may be furnished after 1 April 2026.

This is different from a tax audit for Tax Year 2026–27, for which the new Form 26 applies and the corresponding audit report will be due in 2027.

September 2026 and ITR Filing

The ITR deadline itself depends on the taxpayer category.

For example, the standard non-audit deadline for certain business and professional taxpayers for AY 2026–27 was 31 August 2026.

The Income Tax Department confirms that ITR-4 for eligible taxpayers for AY 2026–27 had a 31 August 2026 due date.

For taxpayers whose accounts are subject to tax audit, the ITR deadline is generally later, with the tax audit report preceding it.

Therefore, September should not be viewed as a universal ITR-filing month.

Instead, taxpayers should determine their applicable return and audit category.

TDS Returns and September Compliance

Apart from depositing TDS, deductors must also ensure that their quarterly TDS statements are prepared correctly.

For Tax Year 2026–27, the Income Tax Department has introduced new forms and provisions under the Income Tax Act, 2025.

For example, Form 140, earlier known as Form 26Q, is a quarterly statement for TDS on non-salary payments to residents.

Businesses should therefore maintain proper records of:

• Deductee PAN
• Nature of payment
• Amount paid
• TDS rate
• TDS deducted
• TDS deposited
• Challan details

Accurate monthly records make quarterly TDS reporting significantly easier.

September Compliance for Businesses

For businesses, September is not just about filing forms.

It is an opportunity to conduct a broader compliance review.

Review Your Books

Ensure sales, purchases, expenses, receivables and payables are properly recorded.

Reconcile GST

Compare books with GSTR-1, GSTR-3B and available GST data.

Review TDS

Check whether TDS was deducted and deposited correctly.

Recalculate Advance Tax

Update your projected annual income and tax liability before the September instalment.

Prepare Audit Records

Businesses subject to tax audit should ensure that financial statements, ledgers, supporting documents and tax records are organised well before the 30 September audit-report deadline.

Common September 2026 Tax Compliance Mistakes

Mistake 1: Missing the Advance Tax Calculation

Taxpayers sometimes remember the 15 September deadline but fail to calculate the required cumulative amount correctly.

Mistake 2: Ignoring TDS Credits

TDS and TCS should be properly considered while estimating advance tax.

Mistake 3: Filing GSTR-1 Without Reconciliation

Uploading sales data without reconciling books can lead to discrepancies.

Mistake 4: Treating GSTR-3B as a Simple Summary

GSTR-3B involves actual GST liability and ITC reporting. Errors can have financial consequences.

Mistake 5: Starting the Tax Audit Too Late

Waiting until September to organise all audit documentation can create unnecessary pressure.

Mistake 6: Confusing Old and New Income-Tax Sections

Tax Year 2026–27 is governed by the new Income Tax Act, 2025, while FY 2025–26 / AY 2026–27 audit reporting continues under the old Act.

Understanding this transition is particularly important for businesses and professionals.

September 2026 Compliance Checklist

Before the month ends, businesses should ideally confirm that they have reviewed:

Income Tax

Advance tax calculation and September instalment.

TDS

August TDS payment and applicable TDS reporting requirements.

GST

GSTR-1 and GSTR-3B preparation, reconciliation and filing.

Tax Audit

Audit documentation and tax-audit report where applicable.

Financial Records

Books of accounts, invoices, bank statements and supporting documents.

Tax Credits

TDS/TCS and other tax payments reflected correctly in available records.

Why Tax Compliance Should Be Planned in Advance

Tax compliance is much easier when deadlines are treated as part of a monthly financial routine rather than as isolated events.

For businesses, a monthly compliance review can help identify errors before they become larger problems.

For example, an incorrect invoice identified before GSTR-1 filing is generally easier to deal with than discovering the issue months later during reconciliation.

Similarly, updating an advance-tax estimate in September gives taxpayers more time to adjust their December and March payments.

How a Chartered Accountant Can Help

A Chartered Accountant can help businesses coordinate different tax obligations instead of handling each compliance requirement separately.

Professional assistance can include:

• Advance-tax calculation
• GST return filing
• GST reconciliation
• TDS compliance
• Tax audit
• Financial statement preparation
• ITR filing
• Tax planning
• Compliance review

This becomes particularly useful for businesses with multiple GST registrations, substantial transactions, professional income or complex tax structures.

Conclusion

September 2026 is a critical month for tax compliance in India.

From the 7 September TDS payment deadline to the 11 September GSTR-1 deadline, 15 September advance-tax instalment, 20 September GSTR-3B deadline and 30 September tax-audit report deadline, businesses and taxpayers have several important obligations to manage.

The most important step is to avoid looking at these deadlines individually.

TDS, GST, advance tax, accounting records and tax audit information are interconnected. A discrepancy in one area can often create complications in another.

For Tax Year 2026–27, taxpayers should also be aware of the transition to the Income Tax Act, 2025, while remembering that the FY 2025–26 tax-audit report remains governed by the Income Tax Act, 1961 and uses Forms 3CA/3CB/3CD.

Planning compliance before each deadline can help businesses avoid unnecessary late fees, interest, mismatches and notices.

Frequently Asked Questions

What are the major tax deadlines in September 2026?

Major September deadlines include 7 September for regular TDS payment, 11 September for monthly GSTR-1, 15 September for the second advance-tax instalment, 20 September for monthly GSTR-3B and 30 September for applicable tax-audit reports.

What is the advance-tax deadline in September 2026?

The second advance-tax instalment is due on 15 September 2026, with eligible taxpayers generally required to reach 45% cumulative payment of estimated annual advance tax.

How much advance tax must be paid by 15 September?

The cumulative payment requirement is generally 45% of the estimated annual advance-tax liability, after considering advance tax already paid in June.

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.

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.

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 and applicable rules.

Is 30 September 2026 the tax-audit deadline?

Yes, for applicable FY 2025–26 / AY 2026–27 cases, the tax-audit report is due on 30 September 2026 where the corresponding ITR due date is 31 October 2026.

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

For FY 2025–26 / AY 2026–27, tax-audit reports continue to use Forms 3CA, 3CB and 3CD under the old Income Tax Act, 1961.

Does the new Income Tax Act, 2025 affect September 2026 compliance?

Yes. Tax Year 2026–27 is governed by the Income Tax Act, 2025. However, certain FY 2025–26 / AY 2026–27 compliance matters, such as the tax-audit report, continue under the old Act.

Can GST due dates be extended?

Yes. The Government can extend GST filing deadlines through notifications. Therefore, taxpayers should check the GST Portal for any extension applicable to a particular tax period.

What happens if a tax deadline is missed?

Depending on the particular compliance, late filing or payment can result in late fees, interest, penalties or other compliance consequences. The exact consequence depends on the return, tax and circumstances involved.

Why is September important for business owners?

September combines several important obligations involving TDS, GST, advance tax and tax audit. Coordinating these requirements can help businesses maintain accurate records and avoid avoidable compliance issues.

Blog By : CA Junaid Khan

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