import tkinter as tk
from tkinter import *
import time
import datetime

root = Tk()
root.attributes("-fullscreen", True)

time1 = 0
clock = Label(root, font=('helvetica', 180, 'bold'), bg='black', fg='white')
clock.pack(fill=BOTH, expand=1)

cronometro = Label(root, font=('helvetica', 140, 'bold'), bg='black', fg='white')
cronometro.pack(fill=BOTH, expand=1)


def tick():
    global time1
    # Pegar horario do PC
    time2 = time.strftime('%H:%M:%S')
    # Se o tempo mudar, atualizar
    if time2 != time1:
        time1 = time2
        clock.config(text=time2)
    root.after(50, tick)
tick()

def countdown(count):
    cronometro['text'] = count

    if count > 0:
        root.after(1000, countdown, count-1)

countdown(70)

root.mainloop(  )