diff --git a/lib/Service/WorkflowRunner.php b/lib/Service/WorkflowRunner.php
index 18f9707..284f638 100644
--- a/lib/Service/WorkflowRunner.php
+++ b/lib/Service/WorkflowRunner.php
@@ -4,7 +4,9 @@ declare(strict_types=1);
 
 namespace OCA\WorkflowDeckAutomation\Service;
 
+use DateTimeImmutable;
 use DateTimeInterface;
+use DateTimeZone;
 use OCA\Deck\Db\Card;
 use OCA\WorkflowDeckAutomation\Db\Workflow;
 use OCA\WorkflowDeckAutomation\Db\WorkflowMapper;
@@ -325,9 +327,31 @@ class WorkflowRunner {
 	 * every card for up to 24h after it actually fell due; comparing the raw datetime
 	 * instead matches what Deck's own UI shows (e.g. "vor 2 Stunden") and has no
 	 * day-rounding step to get wrong.
+	 *
+	 * Both sides are compared as absolute instants, and the due date is first
+	 * pinned to UTC via asStoredUtc() — see there for why.
 	 */
 	public static function isOverdue(?DateTimeInterface $duedate, DateTimeInterface $now): bool {
-		return $duedate !== null && $duedate < $now;
+		return $duedate !== null && self::asStoredUtc($duedate) < $now;
+	}
+
+	/**
+	 * Deck's `duedate` is a DATETIME column without a zone that holds UTC
+	 * wall-clock time: Deck's frontend sends `toISOString()`, and the value is
+	 * written as plain `Y-m-d H:i:s`. Entity hydrates it with
+	 * `new \DateTime($string)`, i.e. in PHP's *default* timezone — correct only
+	 * because Nextcloud sets that default to UTC during bootstrap. Reading the
+	 * wall clock and pinning it to UTC explicitly makes the comparison
+	 * independent of that default, so a server or CLI running with a local
+	 * default zone cannot shift every due date by its UTC offset.
+	 *
+	 * Deck itself has two timezone-sensitive paths this deliberately avoids:
+	 * Card::getDaysUntilDue() (day-truncated, see isOverdue()) and
+	 * CardMapper::findOverdue(), which compares the UTC column against the
+	 * database's `NOW()` — the DB session's local time.
+	 */
+	private static function asStoredUtc(DateTimeInterface $duedate): DateTimeImmutable {
+		return new DateTimeImmutable($duedate->format('Y-m-d H:i:s.u'), new DateTimeZone('UTC'));
 	}
 
 	/**
diff --git a/tests/Unit/Service/WorkflowRunnerTest.php b/tests/Unit/Service/WorkflowRunnerTest.php
index 75870de..f326f05 100644
--- a/tests/Unit/Service/WorkflowRunnerTest.php
+++ b/tests/Unit/Service/WorkflowRunnerTest.php
@@ -5,22 +5,27 @@ declare(strict_types=1);
 namespace OCA\WorkflowDeckAutomation\Tests\Unit\Service;
 
 use DateTimeImmutable;
+use DateTimeZone;
 use OCA\WorkflowDeckAutomation\Service\WorkflowRunner;
 use PHPUnit\Framework\TestCase;
 
 class WorkflowRunnerTest extends TestCase {
+	private static function utc(string $time): DateTimeImmutable {
+		return new DateTimeImmutable($time, new DateTimeZone('UTC'));
+	}
+
 	public function testIsOverdueReturnsFalseWithoutDueDate(): void {
-		$this->assertFalse(WorkflowRunner::isOverdue(null, new DateTimeImmutable('2026-01-15 12:00:00')));
+		$this->assertFalse(WorkflowRunner::isOverdue(null, self::utc('2026-01-15 12:00:00')));
 	}
 
 	public function testIsOverdueReturnsFalseForFutureDueDate(): void {
-		$now = new DateTimeImmutable('2026-01-15 12:00:00');
+		$now = self::utc('2026-01-15 12:00:00');
 		$this->assertFalse(WorkflowRunner::isOverdue($now->modify('+1 hour'), $now));
 		$this->assertFalse(WorkflowRunner::isOverdue($now->modify('+3 days'), $now));
 	}
 
 	public function testIsOverdueReturnsTrueForPastDueDate(): void {
-		$now = new DateTimeImmutable('2026-01-15 12:00:00');
+		$now = self::utc('2026-01-15 12:00:00');
 		$this->assertTrue(WorkflowRunner::isOverdue($now->modify('-1 hour'), $now));
 		$this->assertTrue(WorkflowRunner::isOverdue($now->modify('-30 days'), $now));
 	}
@@ -31,7 +36,32 @@ class WorkflowRunnerTest extends TestCase {
 		// "0 days until due", not overdue, until the calendar date rolls
 		// over - holding every card back for up to 24h after it actually
 		// fell due. isOverdue() must compare the real timestamps instead.
-		$now = new DateTimeImmutable('2026-01-15 12:00:00');
-		$this->assertTrue(WorkflowRunner::isOverdue(new DateTimeImmutable('2026-01-15 06:00:00'), $now));
+		$now = self::utc('2026-01-15 12:00:00');
+		$this->assertTrue(WorkflowRunner::isOverdue(self::utc('2026-01-15 06:00:00'), $now));
+	}
+
+	public function testIsOverdueReadsTheStoredWallClockAsUtc(): void {
+		// What Entity hydration produces if PHP's default zone is not UTC:
+		// the stored UTC wall clock "12:00", tagged with the local zone.
+		$due = new DateTimeImmutable('2026-07-15 12:00:00', new DateTimeZone('Europe/Berlin'));
+
+		// Taken at face value that would be 10:00 UTC and already overdue.
+		$this->assertFalse(WorkflowRunner::isOverdue($due, self::utc('2026-07-15 11:30:00')));
+		$this->assertTrue(WorkflowRunner::isOverdue($due, self::utc('2026-07-15 12:00:01')));
+	}
+
+	public function testIsOverdueComparesNowAsAnAbsoluteInstant(): void {
+		$due = self::utc('2026-07-15 12:00:00');
+		$berlin = new DateTimeZone('Europe/Berlin');
+
+		// 13:30 in Berlin (CEST) is 11:30 UTC - not yet due.
+		$this->assertFalse(WorkflowRunner::isOverdue($due, new DateTimeImmutable('2026-07-15 13:30:00', $berlin)));
+		// 14:30 in Berlin is 12:30 UTC - due.
+		$this->assertTrue(WorkflowRunner::isOverdue($due, new DateTimeImmutable('2026-07-15 14:30:00', $berlin)));
+	}
+
+	public function testIsOverdueIsFalseAtTheExactDueInstant(): void {
+		$now = self::utc('2026-01-15 12:00:00');
+		$this->assertFalse(WorkflowRunner::isOverdue($now, $now));
 	}
 }