myDATA E-Invoicing Compliance: Deadlines & Requirements for 2025–2026

Greece's myDATA (my Digital Accounting and Tax Application) platform represents one of the most ambitious digital tax transformation initiatives in Europe. Operated by AADE (the Independent Authority for Public Revenue), myDATA is progressively mandating real-time electronic transmission of all business transactions. With the March 2026 B2B e-invoicing mandate approaching, businesses must act now to ensure compliance.

What is myDATA?

myDATA is a real-time digital reporting system that requires businesses to electronically transmit transaction data — including income, expenses, and adjustments — directly to AADE's digital platform. The system creates a comprehensive digital record of every business transaction in Greece.

Core Components

• Income Classification: All revenue transactions must be classified and transmitted with standardized type codes.
• Expense Classification: Business expenses must be matched and categorized according to AADE's taxonomy.
• E-invoicing: Progressive move toward fully electronic invoice issuance, transmission, and receipt.
• Real-time Reporting: Transactions must be reported as they occur, not at month-end or quarter-end.

Key Deadlines: What's Changing in 2025–2026

Already in Effect (2024–2025)

• Income transmission: All businesses must transmit income data (revenue invoices) to myDATA in real-time or within the same day.
• Expense classification: Businesses must classify and accept/reject expenses transmitted by their suppliers.
• Retail (B2C): Point-of-sale transactions are already transmitted in real-time via connected cash registers or POS systems.

March 2026: Mandatory B2B E-Invoicing

The most significant upcoming change is the mandatory use of certified e-invoicing providers for all B2B transactions, effective March 2026. This means:

• All invoices between businesses must be issued, transmitted, and received electronically through AADE-certified providers.
• Paper invoices and manually-transmitted PDFs will no longer satisfy compliance requirements for B2B transactions.
• Invoices must be transmitted in real-time (or within a very short window) and bear a unique AADE authentication mark (ΜΑΡΚ).

Penalties for Non-Compliance

AADE has established a structured penalty framework that escalated significantly in 2025:

Violation	Penalty
Failure to transmit income documents	10% of net value per document, capped at €250/day
Late transmission (income)	5% of net value per document, capped at €125/day
Incorrect classification/coding	€250 per instance
Failure to classify expenses	€250 per document
Non-issuance of e-invoice (post-mandate)	Up to 50% of the invoice value
Systematic non-compliance	Additional penalties + potential audit trigger

Note: Penalties may be adjusted. AADE may apply transitional leniency during the first months of new mandates, but this is not guaranteed.

How myDATA Transmission Works

There are three main methods for transmitting data to myDATA:

1. ERP Integration (API): Direct integration between your accounting/ERP software and AADE's myDATA API. This is the recommended approach for businesses with significant transaction volumes.
   • Uses REST APIs with XML/JSON payloads
   • Supports real-time and batch transmission
   • Requires software certification by AADE
2. Certified E-invoicing Providers: Third-party providers (e.g., Retail@Link, SoftOne, Epsilon Net) that handle transmission on your behalf. Mandatory for B2B from March 2026.
   • Provider issues, transmits, and stores invoices
   • Automatically generates the ΜΑΡΚ authentication
   • Handles compliance with format and timing requirements
3. AADE Web Portal: Manual entry through the myDATA online platform. Suitable only for very small businesses with minimal transactions.
   • Free to use
   • Time-consuming for high volumes
   • Will not satisfy B2B e-invoicing requirements after March 2026

Implementation Checklist

To prepare for the March 2026 mandate, businesses should:

1. Audit your current setup
   • Are you currently transmitting all income to myDATA?
   • Are you classifying received expenses?
   • What software do you use for invoicing?
2. Choose a certified e-invoicing provider
   • Evaluate providers based on your transaction volume, ERP compatibility, and cost
   • Major providers in Greece: Epsilon Net, SoftOne, Entersoft, Retail@Link, IMPACT
   • Start the onboarding process at least 3 months before the mandate
3. Update your accounting software
   • Ensure your ERP/accounting system supports myDATA integration
   • Verify it can produce invoices in the required electronic format
   • Test the connection with your provider's sandbox environment
4. Train your team
   • Accounting staff must understand the new classification codes
   • Invoice issuers need training on the e-invoicing workflow
   • Establish procedures for expense classification and reconciliation
5. Test and validate
   • Run parallel testing (paper + electronic) before the mandate date
   • Verify all document types are correctly transmitted
   • Confirm ΜΑΡΚ numbers are generated and stored

Common Mistakes to Avoid

• Waiting until the last minute: Provider onboarding and ERP integration take time. Start now.
• Ignoring expense classification: Many businesses transmit income but neglect to classify expenses received from suppliers. Both are mandatory.
• Mismatched document types: Using incorrect invoice type codes is a common error that triggers penalties.
• Not reconciling myDATA with your books: Your internal accounting records must match what's on the myDATA platform. Regular reconciliation is essential.

How Apostolou & Partners Can Help

We provide end-to-end myDATA compliance support:

• Compliance audit: We assess your current setup and identify gaps.
• Provider selection: We help you choose and onboard the right e-invoicing provider.
• Ongoing management: We handle your myDATA classifications, reconciliation, and reporting.
• Training: We train your team on proper procedures and classification codes.

Don't wait for the mandate to catch you off guard. Contact Apostolou & Partners today for a free myDATA compliance assessment.