diff --git a/src/Chronos.php b/src/Chronos.php
index ada7963..635c109 100644
--- a/src/Chronos.php
+++ b/src/Chronos.php
@@ -694,9 +694,11 @@ protected static function applyTestNowToMissingComponents(
         $hasMicro = (bool)array_intersect($formatChars, ['u', 'v']);
 
         // If the format includes '!' or '|', PHP resets unspecified components to Unix epoch or zero
-        // In that case, we should not override with testNow
+        // If 'U' is present, all components are set from the Unix timestamp
+        // In these cases, we should not override with testNow
         $hasReset = in_array('!', $formatChars, true) || in_array('|', $formatChars, true);
-        if ($hasReset) {
+        $hasUnixTimestamp = in_array('U', $formatChars, true);
+        if ($hasReset || $hasUnixTimestamp) {
             return $dateTime;
         }
 
diff --git a/tests/TestCase/DateTime/CreateFromFormatTest.php b/tests/TestCase/DateTime/CreateFromFormatTest.php
index 91318db..bdf826b 100644
--- a/tests/TestCase/DateTime/CreateFromFormatTest.php
+++ b/tests/TestCase/DateTime/CreateFromFormatTest.php
@@ -111,6 +111,22 @@ public function testCreateFromFormatWithTestNowMicroseconds()
         $this->assertSame(123456, $d->micro);
     }
 
+    public function testCreateFromFormatWithTestNowUnixTimestamp()
+    {
+        // Unix timestamp ('U' format) sets all components, should not use testNow
+        Chronos::setTestNow(new Chronos('2020-12-01 14:30:45'));
+        $d = Chronos::createFromFormat('U', '0');
+        $this->assertDateTime($d, 1970, 1, 1, 0, 0, 0);
+    }
+
+    public function testCreateFromFormatWithTestNowNegativeUnixTimestamp()
+    {
+        // Negative Unix timestamp should also not use testNow
+        Chronos::setTestNow(new Chronos('2020-12-01 14:30:45'));
+        $d = Chronos::createFromFormat('U', '-1000');
+        $this->assertDateTime($d, 1969, 12, 31, 23, 43, 20);
+    }
+
     public function testCreateFromFormatWithTimezoneString()
     {
         $d = Chronos::createFromFormat('Y-m-d H:i:s', '1975-05-21 22:32:11', 'Europe/London');