change update method with time.sleep into after method

[Pi]

Hi, im writing app in Python 3.4 using tkinter. Its timer for windows 7 and higher. Input some minutes and press start button. When time reaches zero it will shutdown computer. Ive used update method with time.sleep - look at my code. But i need to make this timer with after method and i dont know how. root.update and time.sleep gives me TclError when X button is pressed (root.destroy). Ive tried many times without correct answer. Can You help me with my problem?

import time, os
import tkinter as tk
from tkinter import *

def startCount():
    setTime = setMinutes.get() * 60
    strTime = str(setTime)
    timeSet = ("\""+"shutdown /s /f /t " +strTime+"\"")
    os.system(timeSet)
    for t in range(setTime, -1, -1):
        lcd = "{:02d}:{:02d}".format(*divmod(t, 60))
        timeString.set(lcd)
        try:
            root.update()
        except TclError:
            messagebox.showinfo('Info', 'Time is unstoppable.')
            return
        time.sleep(1)
    return

root = tk.Tk()
setMinutes = IntVar()
timeString = StringVar()
label_font = ('Verdana', 30)

L1 = tk.Label(root, text='How much time you have?', anchor='center')
L1.grid(row=0, columnspan=4, sticky='WE')

L2 = Label(root, textvariable=timeString, font=label_font, bg='white', 
         fg='orange', relief='raised', bd=3)
L2.grid(row=1, columnspan=4, sticky='WE', padx=5, pady=5)

E1 = Entry(root, textvariable=setMinutes)
E1.grid(row=2, column=0, columnspan=2, sticky='WE', padx=5, pady=5)

B1 = Button(root, text='S T A R T', fg='green', bg='black', command=startCount)
B1.grid(row=2, column=3, padx=5, pady=5)

root.mainloop()

I tried to add this code with

setTime = setMinutes.get() * 60

as global and I put root.after call one code line before root.mainloop

def displayCountdown():
    global setTime
    for t in range (setTime, -1, -1):
        lcd = "{:02d}:{:02d}".format(*divmod(t, 60))
        timeString.set(lcd)
        setTime -= 1
        if setTime > 0:
            root.after(1000,displayCountdown)

For this moment i do not know how do it correctly, so once again - Please help me

[Gribouillis]

You could use the technique I described in this post https://www.daniweb.com/programming/software-development/threads/505937/python-tkinter-delay-hangs-using-after-method#post2209392