Kivy Object ref

from kivy.app import App
from kivy.clock import Clock
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.lang import Builder
import time

Builder.load_string("""

ScreenManager:
    MyScreen:
        HomeMenu:

<HomeMenu>:
    orientation: "vertical"
    Label:
        id: my_clock
        text: "00:00:00"
        font_size: 30

    """)


class HomeMenu(BoxLayout):

    clock = ObjectProperty()


class MyScreen(Screen):
    pass


class ScreenManager(ScreenManager):
    pass


class TestApp(App):

    def on_start(self):
        Clock.schedule_interval(self.update,1)

    def update(self, nap):
        home = HomeMenu()
        home.clock.text = time.strftime("%H:%M:%S")

if __name__ == "__main__":
    TestApp().run()