diff --git a/ext/standard/JitDateMutation.php b/ext/standard/JitDateMutation.php
index 340367e1679..22fc63c7dc5 100644
--- a/ext/standard/JitDateMutation.php
+++ b/ext/standard/JitDateMutation.php
@@ -728,6 +728,14 @@ private static function invokeObjectIntervalMutationForFunction(
                     DateTimeSupport::MICROSECOND_PROPERTY,
                     $i64->constInt($updated['microsecond'], true)
                 );
+                // format()/getTimestamp() prefer compileTimeDateTimeTimestamp (#33911).
+                // Refresh the stamp + dateTimeLocalInstants or the mutation is a silent no-op.
+                self::publishMutableDateTimeInstant(
+                    $context,
+                    $args[0],
+                    $updated['timestamp'],
+                    $instant['timezone']
+                );
             }
 
             return self::boxObjectPtr($context, $dtObj);
@@ -787,12 +795,19 @@ private static function invokeObjectIntervalMutationForFunction(
                 );
             }
             self::writeLongProp($context, $object, $dtObj, $layout, DateTimeSupport::TS_PROPERTY, $newTs);
+            if (!$immutable) {
+                // Runtime delta — drop stale construct stamp so format reads the slot (#33911).
+                self::invalidateMutableDateTimeInstant($context, $args[0]);
+            }
 
             return self::boxObjectPtr($context, $dtObj);
         }
 
         if (null !== $intervalState) {
             self::applyKnownIntervalStateViaLlvm($context, $object, $dtObj, $intervalState, $layout, $add);
+            if (!$immutable) {
+                self::invalidateMutableDateTimeInstant($context, $args[0]);
+            }
 
             return self::boxObjectPtr($context, $dtObj);
         }
@@ -809,10 +824,52 @@ private static function invokeObjectIntervalMutationForFunction(
             DateMutationRuntime::ensureLinked($context);
             self::applyIntervalToDateTimeObject($context, $dtObj, $intervalObj, $layout, $add);
         }
+        if (!$immutable) {
+            self::invalidateMutableDateTimeInstant($context, $args[0]);
+        }
 
         return self::boxObjectPtr($context, $dtObj);
     }
 
+    /**
+     * Keep format()/getTimestamp() compile-time stamps in sync after mutable add/sub (#33911).
+     */
+    private static function publishMutableDateTimeInstant(
+        Context $context,
+        JITVariable $receiver,
+        int $timestamp,
+        string $timezone
+    ): void {
+        $receiver->compileTimeDateTimeTimestamp = $timestamp;
+        $receiver->compileTimeTimezoneName = $timezone;
+        foreach ($context->namedVariableBindings as $boundName => $bound) {
+            if ($bound !== $receiver) {
+                continue;
+            }
+            $bound->compileTimeDateTimeTimestamp = $timestamp;
+            $bound->compileTimeTimezoneName = $timezone;
+            $context->dateTimeLocalInstants[$boundName] = [
+                'timestamp' => $timestamp,
+                'timezone' => $timezone,
+            ];
+        }
+    }
+
+    /**
+     * Drop construct-time stamps after a runtime LLVM add/sub so reads use __dt_timestamp (#33911).
+     */
+    private static function invalidateMutableDateTimeInstant(Context $context, JITVariable $receiver): void
+    {
+        $receiver->compileTimeDateTimeTimestamp = null;
+        foreach ($context->namedVariableBindings as $boundName => $bound) {
+            if ($bound !== $receiver) {
+                continue;
+            }
+            $bound->compileTimeDateTimeTimestamp = null;
+            unset($context->dateTimeLocalInstants[$boundName]);
+        }
+    }
+
     private static function boxObjectPtr(Context $context, Value $dtObj): Value
     {
         $ret = JitValueBox::alloc($context);
diff --git a/test/repro/issue_33911_datetime_add_gettimestamp.php b/test/repro/issue_33911_datetime_add_gettimestamp.php
new file mode 100644
index 00000000000..6692d14458a
--- /dev/null
+++ b/test/repro/issue_33911_datetime_add_gettimestamp.php
@@ -0,0 +1,6 @@
+<?php
+$d = new DateTime('2020-01-15');
+echo 'before=', $d->getTimestamp(), "\n";
+$d->add(new DateInterval('P1M'));
+echo 'after_ts=', $d->getTimestamp(), "\n";
+echo 'after_fmt=', $d->format('Y-m-d'), "\n";
diff --git a/test/repro/issue_33911_datetime_add_sub_p1m.php b/test/repro/issue_33911_datetime_add_sub_p1m.php
new file mode 100644
index 00000000000..2a0d2b03d4f
--- /dev/null
+++ b/test/repro/issue_33911_datetime_add_sub_p1m.php
@@ -0,0 +1,13 @@
+<?php
+$d = new DateTime('2020-01-15');
+$d->add(new DateInterval('P1M'));
+echo 'add=', $d->format('Y-m-d'), "\n";
+
+$d2 = new DateTime('2020-03-15');
+$d2->sub(new DateInterval('P1M'));
+echo 'sub=', $d2->format('Y-m-d'), "\n";
+
+$i = DateInterval::createFromDateString('1 day + 2 hours');
+$dt = new DateTime('2020-01-01 00:00:00');
+$dt->add($i);
+echo 'cfd=', $dt->format('Y-m-d H:i'), "\n";
diff --git a/test/unit/Issue33911DateTimeAddSubStampAotTest.php b/test/unit/Issue33911DateTimeAddSubStampAotTest.php
new file mode 100644
index 00000000000..cdbba0dac26
--- /dev/null
+++ b/test/unit/Issue33911DateTimeAddSubStampAotTest.php
@@ -0,0 +1,86 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPCompiler\Test\Unit;
+
+use PHPUnit\Framework\TestCase;
+
+/**
+ * AOT DateTime::add/sub must refresh compile-time stamps so format/getTimestamp match Zend (#33911).
+ *
+ * @group llvm
+ * @group aot
+ */
+final class Issue33911DateTimeAddSubStampAotTest extends TestCase
+{
+    public function testMutationPublishesOrInvalidatesCompileTimeStamp(): void
+    {
+        $source = (string) file_get_contents(
+            dirname(__DIR__, 2).'/ext/standard/JitDateMutation.php'
+        );
+        $this->assertStringContainsString('publishMutableDateTimeInstant', $source);
+        $this->assertStringContainsString('invalidateMutableDateTimeInstant', $source);
+        $this->assertStringContainsString('#33911', $source);
+    }
+
+    public function testAotAddSubP1mMatchesZend(): void
+    {
+        if (!\PHPCompiler\LlvmToolchain::hasLibrary(dirname(__DIR__, 2))) {
+            $this->markTestSkipped('LLVM 9 toolchain not available');
+        }
+        $root = dirname(__DIR__, 2);
+        $src = $root.'/test/repro/issue_33911_datetime_add_sub_p1m.php';
+        $bin = sys_get_temp_dir().'/phpc_addsub_33911_'.getmypid().'.bin';
+        $compile = 'PHP_COMPILER_HELPER_RUNTIME_O=0 '.escapeshellarg(PHP_BINARY).' '
+            .escapeshellarg($root.'/bin/compile.php')
+            .' -o '.escapeshellarg($bin).' '.escapeshellarg($src).' 2>&1';
+        exec($compile, $compileOut, $compileRc);
+        $this->assertSame(0, $compileRc, implode("\n", $compileOut));
+
+        $zendOut = [];
+        exec(escapeshellarg(PHP_BINARY).' '.escapeshellarg($src).' 2>&1', $zendOut, $zendRc);
+        $this->assertSame(0, $zendRc, implode("\n", $zendOut));
+        $expected = implode("\n", $zendOut)."\n";
+
+        try {
+            for ($i = 0; $i < 5; ++$i) {
+                $runOut = [];
+                exec(escapeshellarg($bin).' 2>&1', $runOut, $runRc);
+                $this->assertSame(0, $runRc, 'run '.($i + 1).': '.implode("\n", $runOut));
+                $this->assertSame($expected, implode("\n", $runOut)."\n", 'run '.($i + 1));
+            }
+        } finally {
+            @unlink($bin);
+        }
+    }
+
+    public function testAotAddGetTimestampMatchesZend(): void
+    {
+        if (!\PHPCompiler\LlvmToolchain::hasLibrary(dirname(__DIR__, 2))) {
+            $this->markTestSkipped('LLVM 9 toolchain not available');
+        }
+        $root = dirname(__DIR__, 2);
+        $src = $root.'/test/repro/issue_33911_datetime_add_gettimestamp.php';
+        $bin = sys_get_temp_dir().'/phpc_addts_33911_'.getmypid().'.bin';
+        $compile = 'PHP_COMPILER_HELPER_RUNTIME_O=0 '.escapeshellarg(PHP_BINARY).' '
+            .escapeshellarg($root.'/bin/compile.php')
+            .' -o '.escapeshellarg($bin).' '.escapeshellarg($src).' 2>&1';
+        exec($compile, $compileOut, $compileRc);
+        $this->assertSame(0, $compileRc, implode("\n", $compileOut));
+
+        $zendOut = [];
+        exec(escapeshellarg(PHP_BINARY).' '.escapeshellarg($src).' 2>&1', $zendOut, $zendRc);
+        $this->assertSame(0, $zendRc, implode("\n", $zendOut));
+        $expected = implode("\n", $zendOut)."\n";
+
+        try {
+            $runOut = [];
+            exec(escapeshellarg($bin).' 2>&1', $runOut, $runRc);
+            $this->assertSame(0, $runRc, implode("\n", $runOut));
+            $this->assertSame($expected, implode("\n", $runOut)."\n");
+        } finally {
+            @unlink($bin);
+        }
+    }
+}