diff --git a/ext/date/php_date.c b/ext/date/php_date.c
index 84fad72948fd4..13c24de305522 100644
--- a/ext/date/php_date.c
+++ b/ext/date/php_date.c
@@ -352,6 +352,7 @@ static HashTable *date_object_get_gc(zend_object *object, zval **table, int *n);
 static HashTable *date_object_get_properties_for(zend_object *object, zend_prop_purpose purpose);
 static HashTable *date_object_get_gc_interval(zend_object *object, zval **table, int *n);
 static HashTable *date_object_get_properties_interval(zend_object *object);
+static HashTable *date_object_get_properties_for_interval(zend_object *object, zend_prop_purpose purpose);
 static HashTable *date_object_get_gc_period(zend_object *object, zval **table, int *n);
 static HashTable *date_object_get_properties_for_timezone(zend_object *object, zend_prop_purpose purpose);
 static HashTable *date_object_get_gc_timezone(zend_object *object, zval **table, int *n);
@@ -1816,6 +1817,7 @@ static void date_register_classes(void) /* {{{ */
 	date_object_handlers_interval.read_property = date_interval_read_property;
 	date_object_handlers_interval.write_property = date_interval_write_property;
 	date_object_handlers_interval.get_properties = date_object_get_properties_interval;
+	date_object_handlers_interval.get_properties_for = date_object_get_properties_for_interval;
 	date_object_handlers_interval.get_property_ptr_ptr = date_interval_get_property_ptr_ptr;
 	date_object_handlers_interval.get_gc = date_object_get_gc_interval;
 	date_object_handlers_interval.compare = date_interval_compare_objects;
@@ -2235,6 +2237,40 @@ static HashTable *date_object_get_properties_interval(zend_object *object) /* {{
 		return props;
 	}
 
+	/* Duplicate the table if it has refcount > 1 to avoid modifying shared data.
+	 * This can happen with circular references (e.g., $obj->prop = $obj). */
+	if (GC_REFCOUNT(props) > 1) {
+		props = zend_array_dup(props);
+	}
+
+	date_interval_object_to_hash(intervalobj, props);
+
+	return props;
+} /* }}} */
+
+static HashTable *date_object_get_properties_for_interval(zend_object *object, zend_prop_purpose purpose) /* {{{ */
+{
+	HashTable *props;
+	php_interval_obj *intervalobj;
+
+	switch (purpose) {
+		case ZEND_PROP_PURPOSE_DEBUG:
+		case ZEND_PROP_PURPOSE_SERIALIZE:
+		case ZEND_PROP_PURPOSE_VAR_EXPORT:
+		case ZEND_PROP_PURPOSE_JSON:
+		case ZEND_PROP_PURPOSE_ARRAY_CAST:
+			break;
+		default:
+			return zend_std_get_properties_for(object, purpose);
+	}
+
+	intervalobj = php_interval_obj_from_obj(object);
+	props = zend_array_dup(zend_std_get_properties(object));
+
+	if (!intervalobj->initialized) {
+		return props;
+	}
+
 	date_interval_object_to_hash(intervalobj, props);
 
 	return props;
diff --git a/ext/date/tests/bug-gh20503.phpt b/ext/date/tests/bug-gh20503.phpt
new file mode 100644
index 0000000000000..037b5d93bfe4e
--- /dev/null
+++ b/ext/date/tests/bug-gh20503.phpt
@@ -0,0 +1,25 @@
+--TEST--
+GH-20503 (Assertion failure with DateInterval and json_encode on circular reference)
+--FILE--
+<?php
+$obj = new DateInterval('P1W');
+$obj->circular = $obj;
+
+// json_encode with circular reference previously caused an assertion failure
+// in debug builds when modifying a HashTable with refcount > 1
+$result = json_encode($obj);
+var_dump($result === false);
+// Error can be either JSON_ERROR_RECURSION or JSON_ERROR_DEPTH depending on detection method
+var_dump(json_last_error() === JSON_ERROR_RECURSION || json_last_error() === JSON_ERROR_DEPTH);
+
+// Also verify array cast works
+$props = (array) $obj;
+var_dump(count($props) > 0);
+var_dump(isset($props['circular']));
+?>
+--EXPECTF--
+Deprecated: Creation of dynamic property DateInterval::$circular is deprecated in %s on line %d
+bool(true)
+bool(true)
+bool(true)
+bool(true)