diff --git a/src/models/InvoiceId.php b/src/models/InvoiceId.php
index 1bf609b..238022b 100644
--- a/src/models/InvoiceId.php
+++ b/src/models/InvoiceId.php
@@ -26,6 +26,7 @@ class InvoiceId extends Model
 
     /**
      * Issue date (FechaExpedicionFactura), format YYYY-MM-DD.
+     * The serializer will automatically convert this to DD-MM-YYYY as required by the AEAT schema.
      * @var string
      */
     public $issueDate;
@@ -39,8 +40,8 @@ public function rules(): array
             [['issuerNif', 'seriesNumber', 'issueDate'], 'required'],
             [['issuerNif', 'seriesNumber', 'issueDate'], 'string'],
             ['issueDate', fn($value): bool|string =>
-                // Checks for format DD-MM-YYYY (simple regex)
-                (preg_match('/^\\d{2}-\\d{2}-\\d{4}$/', (string) $value)) ? true : 'Must be a valid date (YYYY-MM-DD).'],
+                // Checks for format YYYY-MM-DD (ISO 8601)
+                (preg_match('/^\d{4}-\d{2}-\d{2}$/', (string) $value)) ? true : 'Must be a valid date (YYYY-MM-DD).'],
         ];
     }
 
diff --git a/src/models/InvoiceSubmission.php b/src/models/InvoiceSubmission.php
index e5cf23f..72a5cdf 100644
--- a/src/models/InvoiceSubmission.php
+++ b/src/models/InvoiceSubmission.php
@@ -534,8 +534,8 @@ public function rules(): array
                     return true;
                 }
 
-                // Checks for format DD-MM-YYYY (simple regex)
-                return (preg_match('/^\\d{2}-\\d{2}-\\d{4}$/', $value)) ? true : 'Must be a valid date (YYYY-MM-DD).';
+                // Checks for format YYYY-MM-DD (ISO 8601)
+                return (preg_match('/^\d{4}-\d{2}-\d{2}$/', $value)) ? true : 'Must be a valid date (YYYY-MM-DD).';
             }],
             // New rule to validate the issue date format
             ['invoiceId', function ($value): bool|string {
@@ -547,10 +547,10 @@ public function rules(): array
                     return true;
                 }
 
-                // Verifica el formato DD-MM-YYYY
-                return (preg_match('/^\\d{2,2}-\\d{2,2}-\\d{4,4}$/', $value->issueDate))
+                // Verifica el formato YYYY-MM-DD (ISO 8601)
+                return (preg_match('/^\d{4}-\d{2}-\d{2}$/', $value->issueDate))
                     ? true
-                    : 'La fecha de expedición debe tener el formato DD-MM-YYYY.';
+                    : 'The issue date must be in YYYY-MM-DD format.';
             }],
         ]);
     }
diff --git a/src/services/InvoiceSerializer.php b/src/services/InvoiceSerializer.php
index e8615f7..01bbbcd 100644
--- a/src/services/InvoiceSerializer.php
+++ b/src/services/InvoiceSerializer.php
@@ -61,7 +61,7 @@ public static function toInvoiceXml(InvoiceSubmission $invoice, bool $validate =
             $idFactura = $doc->createElementNS(self::SF_NAMESPACE, 'sf:IDFactura');
             $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFactura', (string) $invoiceId->issuerNif));
             $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFactura', (string) $invoiceId->seriesNumber));
-            $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $invoiceId->issueDate));
+            $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $invoiceId->issueDate)));
             $root->appendChild($idFactura);
         }
 
@@ -91,7 +91,7 @@ public static function toInvoiceXml(InvoiceSubmission $invoice, bool $validate =
                 $idFacturaRectificada = $doc->createElementNS(self::SF_NAMESPACE, 'sf:IDFacturaRectificada');
                 $idFacturaRectificada->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFactura', (string) $rect['issuerNif']));
                 $idFacturaRectificada->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFactura', (string) $rect['seriesNumber']));
-                $idFacturaRectificada->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $rect['issueDate']));
+                $idFacturaRectificada->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $rect['issueDate'])));
                 $facturasRectificadas->appendChild($idFacturaRectificada);
             }
             $root->appendChild($facturasRectificadas);
@@ -104,7 +104,7 @@ public static function toInvoiceXml(InvoiceSubmission $invoice, bool $validate =
                 $idFacturaSustituida = $doc->createElementNS(self::SF_NAMESPACE, 'sf:IDFacturaSustituida');
                 $idFacturaSustituida->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFactura', (string) $subst['issuerNif']));
                 $idFacturaSustituida->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFactura', (string) $subst['seriesNumber']));
-                $idFacturaSustituida->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $subst['issueDate']));
+                $idFacturaSustituida->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $subst['issueDate'])));
                 $facturasSustituidas->appendChild($idFacturaSustituida);
             }
             $root->appendChild($facturasSustituidas);
@@ -123,7 +123,7 @@ public static function toInvoiceXml(InvoiceSubmission $invoice, bool $validate =
         }
 
         if (!empty($invoice->operationDate)) {
-            $root->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaOperacion', (string) $invoice->operationDate));
+            $root->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaOperacion', self::formatDate((string) $invoice->operationDate)));
         }
 
         $root->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:DescripcionOperacion', (string) $invoice->operationDescription));
@@ -292,7 +292,7 @@ public static function toInvoiceXml(InvoiceSubmission $invoice, bool $validate =
                     $registroAnterior = $doc->createElementNS(self::SF_NAMESPACE, 'sf:RegistroAnterior');
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFactura', (string) $previousInvoice->issuerNif));
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFactura', (string) $previousInvoice->seriesNumber));
-                    $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $previousInvoice->issueDate));
+                    $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $previousInvoice->issueDate)));
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:Huella', (string) $previousInvoice->hash));
                     $encadenamiento->appendChild($registroAnterior);
                 } else {
@@ -353,7 +353,7 @@ public static function toCancellationXml(InvoiceCancellation $cancellation, bool
             $idFactura = $doc->createElementNS(self::SF_NAMESPACE, 'sf:IDFactura');
             $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFacturaAnulada', (string) $invoiceId->issuerNif));
             $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFacturaAnulada', (string) $invoiceId->seriesNumber));
-            $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFacturaAnulada', (string) $invoiceId->issueDate));
+            $idFactura->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFacturaAnulada', self::formatDate((string) $invoiceId->issueDate)));
             $root->appendChild($idFactura);
         }
 
@@ -403,7 +403,7 @@ public static function toCancellationXml(InvoiceCancellation $cancellation, bool
                     $registroAnterior = $doc->createElementNS(self::SF_NAMESPACE, 'sf:RegistroAnterior');
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:IDEmisorFactura', (string) $previousInvoice->issuerNif));
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:NumSerieFactura', (string) $previousInvoice->seriesNumber));
-                    $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $previousInvoice->issueDate));
+                    $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $previousInvoice->issueDate)));
                     $registroAnterior->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:Huella', (string) $previousInvoice->hash));
                     $encadenamiento->appendChild($registroAnterior);
                 } else {
@@ -508,7 +508,7 @@ public static function toQueryXml(InvoiceQuery $query, bool $validate = true): \
         // FechaExpedicionFactura (Consulta ns) wrapping SF choice type
         if (!empty($query->issueDate)) {
             $fechaWrapper = $doc->createElementNS(self::QUERY_NAMESPACE, 'sf:FechaExpedicionFactura');
-            $fechaWrapper->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', (string) $query->issueDate));
+            $fechaWrapper->appendChild($doc->createElementNS(self::SF_NAMESPACE, 'sf:FechaExpedicionFactura', self::formatDate((string) $query->issueDate)));
             $filtro->appendChild($fechaWrapper);
         }
 
@@ -525,6 +525,25 @@ public static function toQueryXml(InvoiceQuery $query, bool $validate = true): \
         return $doc;
     }
 
+    /**
+     * Converts a date string from YYYY-MM-DD to DD-MM-YYYY format as required by the AEAT schema.
+     * If the value is already in DD-MM-YYYY format, it is returned unchanged.
+     * Any other format is returned as-is; validation of the format should be done prior to calling this method.
+     *
+     * @param string $date Date string in YYYY-MM-DD or DD-MM-YYYY format
+     * @return string Date string in DD-MM-YYYY format
+     */
+    public static function formatDate(string $date): string
+    {
+        if (preg_match('/^\d{4}-\d{2}-\d{2}$/', $date)) {
+            [$year, $month, $day] = explode('-', $date);
+            return "{$day}-{$month}-{$year}";
+        }
+
+        // Already in DD-MM-YYYY format or unrecognized — return unchanged
+        return $date;
+    }
+
     public static function validateXml(\DOMDocument $doc, string $schemaPath): void
     {
         libxml_use_internal_errors(true);
diff --git a/tests/Unit/Services/InvoiceSerializerTest.php b/tests/Unit/Services/InvoiceSerializerTest.php
index 99dba7e..2a3621c 100644
--- a/tests/Unit/Services/InvoiceSerializerTest.php
+++ b/tests/Unit/Services/InvoiceSerializerTest.php
@@ -374,7 +374,7 @@ public function testOtherIdNetherlandsCountryCode(): void
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = '12345678Z';
         $invoiceId->seriesNumber = 'TEST-NL-001';
-        $invoiceId->issueDate = '01-01-2023';
+        $invoiceId->issueDate = '2023-01-01';
         $invoiceNL->setInvoiceId($invoiceId);
         $invoiceNL->issuerName = 'Test Company';
         $invoiceNL->invoiceType = InvoiceType::STANDARD;
@@ -413,7 +413,7 @@ private function createBasicInvoiceSubmission(): InvoiceSubmission
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = '12345678Z';
         $invoiceId->seriesNumber = 'TEST001';
-        $invoiceId->issueDate = '01-01-2023';
+        $invoiceId->issueDate = '2023-01-01';
         $invoice->setInvoiceId($invoiceId);
 
         $invoice->issuerName = 'Test Company';
@@ -452,7 +452,7 @@ private function createBasicInvoiceCancellation(): InvoiceCancellation
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = '12345678Z';
         $invoiceId->seriesNumber = 'TEST001';
-        $invoiceId->issueDate = '01-01-2023';
+        $invoiceId->issueDate = '2023-01-01';
         $cancellation->setInvoiceId($invoiceId);
 
         // Set hash-related properties
@@ -482,7 +482,7 @@ private function createBasicInvoiceQuery(): InvoiceQuery
 
         // Set optional properties
         $query->seriesNumber = 'TEST001';
-        $query->issueDate = '01-01-2023';
+        $query->issueDate = '2023-01-01';
         $query->externalRef = 'TEST-REF-001';
 
         // Set counterparty
diff --git a/tests/Verifactu/ReadmeExamplesTest.php b/tests/Verifactu/ReadmeExamplesTest.php
index 3290861..4282262 100644
--- a/tests/Verifactu/ReadmeExamplesTest.php
+++ b/tests/Verifactu/ReadmeExamplesTest.php
@@ -36,7 +36,7 @@ public function testInvoiceRegistrationExample(): void
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = 'B12345678';
         $invoiceId->seriesNumber = 'FA2024/001';
-        $invoiceId->issueDate = '01-07-2024';
+        $invoiceId->issueDate = '2024-07-01';
         $invoice->setInvoiceId($invoiceId);
 
         // Set basic invoice data
@@ -83,7 +83,7 @@ public function testInvoiceRegistrationExample(): void
         $invoice->hash = '1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef'; // Calculated hash
 
         // Optional fields
-        $invoice->operationDate = '01-07-2024'; // Operation date
+        $invoice->operationDate = '2024-07-01'; // Operation date
         $invoice->externalRef = 'REF123'; // External reference
         $invoice->simplifiedInvoice = YesNoType::NO; // Is not a simplified invoice
         $invoice->invoiceWithoutRecipient = YesNoType::NO; // Has identified recipient
@@ -113,7 +113,7 @@ public function testInvoiceRegistrationExample(): void
         $this->assertInstanceOf(InvoiceId::class, $invoice->getInvoiceId());
         $this->assertEquals('B12345678', $invoice->getInvoiceId()->issuerNif);
         $this->assertEquals('FA2024/001', $invoice->getInvoiceId()->seriesNumber);
-        $this->assertEquals('01-07-2024', $invoice->getInvoiceId()->issueDate);
+        $this->assertEquals('2024-07-01', $invoice->getInvoiceId()->issueDate);
         $this->assertEquals('Empresa Ejemplo SL', $invoice->issuerName);
         $this->assertEquals(InvoiceType::STANDARD, $invoice->invoiceType);
         $this->assertEquals('Venta de productos', $invoice->operationDescription);
@@ -133,7 +133,7 @@ public function testInvoiceCancellationExample(): void
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = 'B12345678';
         $invoiceId->seriesNumber = 'FA2024/001';
-        $invoiceId->issueDate = '01-07-2024';
+        $invoiceId->issueDate = '2024-07-01';
         $cancellation->setInvoiceId($invoiceId);
 
         // Set chaining data (using object-oriented approach)
@@ -142,7 +142,7 @@ public function testInvoiceCancellationExample(): void
         $chaining->setPreviousInvoice([
             'seriesNumber' => 'FA2024/000',
             'issuerNif' => 'B12345678',
-            'issueDate' => '30-06-2024',
+            'issueDate' => '2024-06-30',
             'hash' => '1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef',
         ]);
         $cancellation->setChaining($chaining);
@@ -193,7 +193,7 @@ public function testInvoiceCancellationExample(): void
         $this->assertInstanceOf(InvoiceId::class, $cancellation->getInvoiceId());
         $this->assertEquals('B12345678', $cancellation->getInvoiceId()->issuerNif);
         $this->assertEquals('FA2024/001', $cancellation->getInvoiceId()->seriesNumber);
-        $this->assertEquals('01-07-2024', $cancellation->getInvoiceId()->issueDate);
+        $this->assertEquals('2024-07-01', $cancellation->getInvoiceId()->issueDate);
         $this->assertEquals(YesNoType::NO, $cancellation->noPreviousRecord);
         $this->assertEquals(YesNoType::NO, $cancellation->previousRejection);
         $this->assertEquals(GeneratorType::ISSUER, $cancellation->generator);
diff --git a/tests/Verifactu/VerifactuSandboxTest.php b/tests/Verifactu/VerifactuSandboxTest.php
index 7ec7e3b..e72ead1 100644
--- a/tests/Verifactu/VerifactuSandboxTest.php
+++ b/tests/Verifactu/VerifactuSandboxTest.php
@@ -119,7 +119,7 @@ private function createTestInvoice(): InvoiceSubmission
         $invoiceId = new InvoiceId();
         $invoiceId->issuerNif = $this->issuerNif; // was hardcoded before
         $invoiceId->seriesNumber = 'TEST' . date('YmdHis');
-        $invoiceId->issueDate = date('d-m-Y');
+        $invoiceId->issueDate = date('Y-m-d');
         $invoice->setInvoiceId($invoiceId);
 
         // Set basic invoice data
@@ -168,7 +168,7 @@ private function createTestInvoice(): InvoiceSubmission
         $invoice->recordTimestamp = date('Y-m-d\TH:i:sP');
 
         // Optional fields
-        $invoice->operationDate = date('d-m-Y');
+        $invoice->operationDate = date('Y-m-d');
         // Change to externalReference if you prefer not to depend on the alias:
         // $invoice->externalReference = 'TEST-' . date('YmdHis');
         $invoice->externalRef = 'TEST-' . date('YmdHis');