cronometro python tkinter

import time
from tkinter import *
from tkinter import messagebox
 
 
root = Tk()
root.geometry("300x250")
  
root.title("Time Counter")
hour=StringVar()
minute=StringVar()
second=StringVar()
hour.set("00")
minute.set("00")
second.set("00")
hourEntry= Entry(root, width=3, font=("Arial",18,""),
                 textvariable=hour)
hourEntry.place(x=80,y=20)
  
minuteEntry= Entry(root, width=3, font=("Arial",18,""),
                   textvariable=minute)
minuteEntry.place(x=130,y=20)
  
secondEntry= Entry(root, width=3, font=("Arial",18,""),
                   textvariable=second)
secondEntry.place(x=180,y=20)
  
  
def submit():
    try:
      
        temp = int(hour.get())*3600 + int(minute.get())*60 + int(second.get())
    except:
        print("Please input the right value")
    while temp >-1:
       
        mins,secs = divmod(temp,60) 
      
        hours=0
        if mins >60:
                   hours, mins = divmod(mins, 60)
       
      hour.set("{0:2d}".format(hours))
        minute.set("{0:2d}".format(mins))
        second.set("{0:2d}".format(secs))
      root.update()
        time.sleep(1)
      if (temp == 0):
            messagebox.showinfo("Time Countdown", "Time's up ")
       
      temp -= 1
 
btn = Button(root, text='Set Time Countdown', bd='5',
             command= submit)
btn.place(x = 70,y = 120)
  
root.mainloop()
Marcio Moises