import tkinter as tk
from time import strftime as time

reldigital = tk.Label (text=time('%H:%M:%S'), font='arial 110', fg = 'Blue')
reldigital.pack()
def relogio():
    agora = time('%H:%M:%S')
    if reldigital['text'] != agora:
         reldigital['text'] = agora
    reldigital.after(1000, relogio)
relogio()
reldigital.mainloop()