diff --git a/inc/Blocks/Calendar/Grouping/MultiDayResolver.php b/inc/Blocks/Calendar/Grouping/MultiDayResolver.php
index 71ef2b10..dd76de7a 100644
--- a/inc/Blocks/Calendar/Grouping/MultiDayResolver.php
+++ b/inc/Blocks/Calendar/Grouping/MultiDayResolver.php
@@ -44,6 +44,13 @@ public static function is_multi_day( array $event_data ): bool {
 			return false;
 		}
 
+		// Placeholder / TBD dates (e.g. "2026-07-??") are not parseable and
+		// cannot be expanded into a date range. Treat them as single-day so
+		// the calendar still renders rather than throwing a fatal.
+		if ( ! self::is_valid_date( $start_date ) || ! self::is_valid_date( $end_date ) ) {
+			return false;
+		}
+
 		$start = new DateTime( $start_date );
 		$end   = new DateTime( $end_date );
 		$diff  = $start->diff( $end )->days;
@@ -75,6 +82,17 @@ public static function is_multi_day( array $event_data ): bool {
 	public static function get_date_range( string $start_date, string $end_date, DateTimeZone $event_tz ): array {
 		$dates = array();
 
+		// Guard against placeholder / malformed dates that would throw a
+		// DateMalformedStringException. is_multi_day() already filters these,
+		// but get_date_range() is a public entry point in its own right.
+		if ( ! self::is_valid_date( $start_date ) || ! self::is_valid_date( $end_date ) ) {
+			if ( self::is_valid_date( $start_date ) ) {
+				$dates[] = $start_date;
+			}
+
+			return $dates;
+		}
+
 		$start = new DateTime( $start_date, $event_tz );
 		$end   = new DateTime( $end_date, $event_tz );
 
@@ -89,4 +107,23 @@ public static function get_date_range( string $start_date, string $end_date, Dat
 
 		return $dates;
 	}
+
+	/**
+	 * Strictly validate a Y-m-d date string.
+	 *
+	 * Rejects placeholder / TBD values (e.g. "2026-07-??") and any string
+	 * that does not represent a real calendar date, so callers can avoid
+	 * passing unparseable input into DateTime and triggering a
+	 * DateMalformedStringException.
+	 *
+	 * @param string $date Date string to validate.
+	 * @return bool True when $date is a valid Y-m-d calendar date.
+	 */
+	private static function is_valid_date( string $date ): bool {
+		if ( ! preg_match( '/^(\d{4})-(\d{2})-(\d{2})$/', $date, $matches ) ) {
+			return false;
+		}
+
+		return checkdate( (int) $matches[2], (int) $matches[3], (int) $matches[1] );
+	}
 }
diff --git a/tests/Unit/MultiDayResolverTest.php b/tests/Unit/MultiDayResolverTest.php
new file mode 100644
index 00000000..696980a4
--- /dev/null
+++ b/tests/Unit/MultiDayResolverTest.php
@@ -0,0 +1,105 @@
+<?php
+/**
+ * MultiDayResolver Tests
+ *
+ * Guards against malformed / placeholder dates (e.g. "2026-07-??") that
+ * previously reached `new DateTime()` and threw a fatal
+ * DateMalformedStringException.
+ *
+ * @package DataMachineEvents\Tests\Unit
+ * @since 0.44.1
+ */
+
+namespace DataMachineEvents\Tests\Unit;
+
+use WP_UnitTestCase;
+use DateTimeZone;
+use DataMachineEvents\Blocks\Calendar\Grouping\MultiDayResolver;
+
+class MultiDayResolverTest extends WP_UnitTestCase {
+
+	/**
+	 * Regression: a placeholder end date ("2026-07-??") must not throw.
+	 *
+	 * Previously `is_multi_day()` passed the raw string straight into
+	 * `new DateTime()`, which throws DateMalformedStringException on PHP 8.3+.
+	 */
+	public function test_is_multi_day_handles_placeholder_end_date_without_throwing() {
+		$result = MultiDayResolver::is_multi_day(
+			array(
+				'startDate' => '2026-07-01',
+				'endDate'   => '2026-07-??',
+			)
+		);
+
+		$this->assertFalse( $result, 'Placeholder dates cannot form a range and must be treated as single-day.' );
+	}
+
+	public function test_is_multi_day_handles_placeholder_start_date_without_throwing() {
+		$result = MultiDayResolver::is_multi_day(
+			array(
+				'startDate' => '2026-??-??',
+				'endDate'   => '2026-07-05',
+			)
+		);
+
+		$this->assertFalse( $result );
+	}
+
+	public function test_is_multi_day_returns_false_for_invalid_calendar_dates() {
+		$result = MultiDayResolver::is_multi_day(
+			array(
+				'startDate' => '2026-02-30',
+				'endDate'   => '2026-03-05',
+			)
+		);
+
+		$this->assertFalse( $result );
+	}
+
+	public function test_is_multi_day_true_for_genuine_multi_day_span() {
+		$result = MultiDayResolver::is_multi_day(
+			array(
+				'startDate' => '2026-07-01',
+				'endDate'   => '2026-07-05',
+			)
+		);
+
+		$this->assertTrue( $result );
+	}
+
+	public function test_is_multi_day_false_for_same_day() {
+		$result = MultiDayResolver::is_multi_day(
+			array(
+				'startDate' => '2026-07-01',
+				'endDate'   => '2026-07-01',
+			)
+		);
+
+		$this->assertFalse( $result );
+	}
+
+	public function test_get_date_range_returns_empty_for_malformed_dates() {
+		$tz    = new DateTimeZone( 'America/New_York' );
+		$range = MultiDayResolver::get_date_range( '2026-07-??', '2026-07-??', $tz );
+
+		$this->assertSame( array(), $range );
+	}
+
+	public function test_get_date_range_returns_single_start_when_end_is_malformed() {
+		$tz    = new DateTimeZone( 'America/New_York' );
+		$range = MultiDayResolver::get_date_range( '2026-07-01', '2026-07-??', $tz );
+
+		$this->assertSame( array( '2026-07-01' ), $range );
+	}
+
+	public function test_get_date_range_expands_valid_span() {
+		$tz    = new DateTimeZone( 'America/New_York' );
+		$range = MultiDayResolver::get_date_range( '2026-07-01', '2026-07-03', $tz );
+
+		$this->assertSame(
+			array( '2026-07-01', '2026-07-02', '2026-07-03' ),
+			$range
+		);
+	}
+}