Fix timestamp.

chime/sensor.py

@@ -61,7 +61,7 @@ class Reading:
 
     def json(self) -> bytes:
         return bytes(json.dumps({
-            'timestamp': self.time.timestamp,
+            'timestamp': self.time.timestamp(),
             'temperature': self.temp,
             'pressure': self.press,
             'relative_humidity': self.hum,