diff --git a/src/Events/EventInput.php b/src/Events/EventInput.php
new file mode 100644
index 0000000000..44d2260f64
--- /dev/null
+++ b/src/Events/EventInput.php
@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace phpweb\Events;
+
+final class EventInput
+{
+    /**
+     * The numeric event fields (start/end date parts and recurrence) that must be
+     * integers before being passed to checkdate() or mktime().
+     */
+    private const NUMERIC_FIELDS = [
+        'sday', 'smonth', 'syear', 'eday',
+        'emonth', 'eyear', 'recur', 'recur_day',
+    ];
+
+    /**
+     * Coerce the numeric event fields of a raw request array to integers.
+     *
+     * Untrusted input (non-numeric strings, arrays, missing values) would otherwise
+     * reach checkdate()/mktime() and throw a TypeError under PHP 8. Anything that is
+     * not a numeric value becomes 0, i.e. an invalid date that the normal form
+     * validation already rejects. Non-numeric fields are left untouched.
+     *
+     * @param array<string, mixed> $post
+     * @return array<string, mixed>
+     */
+    public static function normalizeNumericFields(array $post): array
+    {
+        foreach (self::NUMERIC_FIELDS as $field) {
+            $value = $post[$field] ?? null;
+            $post[$field] = is_numeric($value) ? (int) $value : 0;
+        }
+
+        return $post;
+    }
+}
diff --git a/submit-event.php b/submit-event.php
index 2d2ac20044..4809ed9b6e 100644
--- a/submit-event.php
+++ b/submit-event.php
@@ -1,4 +1,6 @@
 <?php
+use phpweb\Events\EventInput;
+
 $_SERVER['BASE_PAGE'] = 'submit-event.php';
 include_once __DIR__ . '/include/prepend.inc';
 include_once __DIR__ . '/include/posttohost.inc';
@@ -9,16 +11,11 @@
 $errors = [];
 $process = [] !== $_POST;
 
+// Coerce the numeric date/recurrence fields to integers so untrusted input can be
+// passed safely to checkdate()/mktime() below instead of throwing a TypeError.
+$_POST = EventInput::normalizeNumericFields($_POST);
+
 // Avoid E_NOTICE errors on incoming vars if not set
-$vars = [
-    'sday', 'smonth', 'syear', 'eday',
-    'emonth', 'eyear', 'recur', 'recur_day',
-];
-foreach ($vars as $varname) {
-    if (empty($_POST[$varname])) {
-        $_POST[$varname] = 0;
-    }
-}
 $vars = [
     'type', 'country', 'category', 'email', 'url', 'ldesc', 'sdesc',
 ];
diff --git a/tests/Unit/Events/EventInputTest.php b/tests/Unit/Events/EventInputTest.php
new file mode 100644
index 0000000000..04d3e9524e
--- /dev/null
+++ b/tests/Unit/Events/EventInputTest.php
@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+namespace phpweb\Test\Unit\Events;
+
+use phpweb\Events\EventInput;
+use PHPUnit\Framework;
+
+#[Framework\Attributes\CoversClass(EventInput::class)]
+final class EventInputTest extends Framework\TestCase
+{
+    public function testCoercesNumericStringsToIntegers(): void
+    {
+        $result = EventInput::normalizeNumericFields(['smonth' => '12', 'sday' => '25', 'syear' => '2026']);
+
+        self::assertSame(12, $result['smonth']);
+        self::assertSame(25, $result['sday']);
+        self::assertSame(2026, $result['syear']);
+    }
+
+    public function testCoercesNonNumericStringsToZero(): void
+    {
+        $result = EventInput::normalizeNumericFields(['smonth' => 'January', 'sday' => 'abc', 'syear' => '12abc']);
+
+        self::assertSame(0, $result['smonth']);
+        self::assertSame(0, $result['sday']);
+        self::assertSame(0, $result['syear']);
+    }
+
+    public function testCoercesArrayValuesToZero(): void
+    {
+        $result = EventInput::normalizeNumericFields(['smonth' => ['x'], 'sday' => [], 'syear' => ['1', '2']]);
+
+        self::assertSame(0, $result['smonth']);
+        self::assertSame(0, $result['sday']);
+        self::assertSame(0, $result['syear']);
+    }
+
+    public function testDefaultsMissingFieldsToZero(): void
+    {
+        $result = EventInput::normalizeNumericFields([]);
+
+        foreach (['sday', 'smonth', 'syear', 'eday', 'emonth', 'eyear', 'recur', 'recur_day'] as $field) {
+            self::assertSame(0, $result[$field], $field);
+        }
+    }
+
+    public function testLeavesNonNumericFieldsUntouched(): void
+    {
+        $result = EventInput::normalizeNumericFields(['email' => 'a@b.com', 'type' => 'multi', 'smonth' => '5']);
+
+        self::assertSame('a@b.com', $result['email']);
+        self::assertSame('multi', $result['type']);
+        self::assertSame(5, $result['smonth']);
+    }
+
+    /**
+     * Regression test for the production fatal:
+     *   Uncaught TypeError: checkdate(): Argument #1 ($month) must be of type int, string given
+     *
+     * After normalization the date parts are always integers, so passing them to
+     * checkdate() (and mktime()) can no longer throw a TypeError; garbage input
+     * simply becomes an invalid (0) date that the normal validation rejects.
+     */
+    public function testNormalizedValuesAreSafeForCheckdate(): void
+    {
+        $result = EventInput::normalizeNumericFields(['smonth' => 'notamonth', 'sday' => '1', 'syear' => '2026']);
+
+        self::assertFalse(checkdate($result['smonth'], $result['sday'], $result['syear']));
+    }
+}