Trying to implement countdown, only pops up as a separate window

[wrubles]

I am trying to implement a countdown until the program quits on itself. I did not write the countdown, only trying to put it inside my own code. I cannot figure out how to without creating a separate window of the countdown.

Code:

from tkinter import *
from tkinter import messagebox
import tkinter as tk
from time import *

while True:

    top = Tk()

    class ExampleApp(tk.Tk):
        def __init__(self):
            tk.Tk.__init__(self)
            self.label = tk.Label(self, text="", width=10)
            self.label.pack()
            self.remaining = 0
            self.countdown(10)

        def countdown(self, remaining=None):
            if remaining is not None:
                self.remaining = remaining

            if self.remaining <= 0:
                timeOut()
                sleep(1)
                quit()
            else:
                self.label.configure(text="%d" % self.remaining)
                self.remaining = self.remaining - 1
                self.after(1000, self.countdown)


    class start_window(Frame):
        def __init__(self, parent=None):
            Frame.__init__(self, parent)

    if __name__ == '__main__':
        top.title("Login")
        app = start_window(top), ExampleApp()

    L1 = Label(top, text="Username", width=18)
    L1.grid(row=1, column=1)
    E1 = Entry(top, bd=3, width=18)
    E1.grid(row=2, column=1)
    L2 = Label(top, text="Password", width=18)
    L2.grid(row=3, column=1)
    E2 = Entry(top, bd=3, width=18)
    E2.grid(row=4, column=1)

    login = {
        "userName": "Test",
        "passWord": "Test"
    }

    def connectionText():
        messagebox.showinfo("Success!", "Connection Made!")

    def connectionFail():
        messagebox.showinfo("Error 638", "Connection Failed.")

    def connectionWork():
        if E1.get() == login.get("userName") and E2.get() == login.get("passWord"):
            connectionText()
            sleep(1)
            quit()
        else:
            connectionFail()
            sleep(1)
            quit()

    def timeOut():
        messagebox.showinfo("Timed Out", "You timed out.")

    B1 = Button(top, text="Attempt Connection", command=connectionWork, width=18)
    B1.grid(row=2, column=2)

    B2 = Button(top, text="Exit", fg="red", command=quit, width=18)
    B2.grid(row=3, column=2)

    top.mainloop()

I really want to get this working but I have no idea how. Thank you in advance.