from tkinter import *
import time

root = Tk()

timeone = ''
clock = Label(root, font=('Tahoma', 48), text='')
clock.pack(fill=BOTH, expand=1)

def tick():
    global timeone
    timetwo = time.strftime('%H:%M:%S')

    if timetwo != timeone:
        timeone = timetwo
        clock.config(text=timetwo)

    clock.after(200, tick)

tick()
root.mainloop()