How do I properly update the countdown timer in one second intervals in the main window?

Question

I'm trying to write a shutdown timer in Python as a newbie programmer. The purpose of the program is to accept a time in a GUI entry box and when the user clicks the shutdown/restart button to start a timer, showing the remaining time in the same window.

Currently, I've got the program to basically function but I don't know how to make the timer update properly (except in a crude print() timer). I see that there are some answers on other programming languages but I'm not yet fluent in those. Also, as I've said, I'm a new programmer so if you see anything else that I could do better I would welcome constructive criticism.

# This program accepts a time value for shutdown/restart
# counts down, shutting down the system at the end of the timer.

import tkinter
import time
import os

class ShutdownTimerGUI:
    def __init__(self):

        # Create the main window.
        self.main_window = tkinter.Tk()

        # Create three frames to group widgets.
        self.top_frame = tkinter.Frame()
        self.mid_frame = tkinter.Frame()
        self.bottom_frame = tkinter.Frame()

        # Create the widgets for the top frame.

        self.prompt_label = tkinter.Label(self.top_frame, \
                                      text='Enter the number of minutes until shutdown:')
        self.time_entry = tkinter.Entry(self.top_frame, \
                                    width=3)

        # Pack the top frame's widgets.
        self.prompt_label.pack(side='left')
        self.time_entry.pack(side='left')

        # Create the widgets for the middle frame.
        self.descr_label = tkinter.Label(self.mid_frame, \
                                     text='Time until shutdown:')

        # Associate a StringVar object with the output label. Use the object's
        # set method to store a string of blank characters.
        self.value = tkinter.StringVar()

        # Create a label and associate it with the StringVar object. Any
        # value stored in the StringVar object will automatically be displayed
        # in the label.
        self.time_label = tkinter.Label(self.mid_frame, \
                                    textvariable=self.value)

        # Pack the middle frame's widgets.
        self.descr_label.pack(side='left')
        self.time_label.pack(side='left')

        # Create the button widgets for the bottom frame.
        self.shutdown_button = tkinter.Button(self.bottom_frame, \
                                          text='Shutdown', \
                                          command=self.shutdown)
        self.restart_button = tkinter.Button(self.bottom_frame, \
                                          text='Restart', \
                                          command=self.restart)
        self.shutdown_button.pack()
        self.restart_button.pack()

        # Pack the buttons.
        self.top_frame.pack()
        self.mid_frame.pack()
        self.bottom_frame.pack()

        #Enter the tkinter main loop.

        tkinter.mainloop()

    # The shutdown method is a callback function for the Shutdown Button.

    def shutdown(self):
        # Get the value entered by the user into the time_entry widget.
        minutes = float(self.time_entry.get())
        seconds = minutes * 60
        count = 0
        self.time_left()
        os.system('shutdown /s')

    # The restart method is a callback function for the Restart Button.

    def restart(self):
        # Get the value entered by the user into the time_entry widget.
        minutes = float(self.time_entry.get())
        seconds = minutes * 60
        count = 0
        self.time_left()
        os.system('shutdown /r')

    def time_left(self):
        minutes = float(self.time_entry.get())
        seconds = minutes * 60
        count = 0
        while count < seconds:
            time.sleep(1)
            count += 1
            time_left = seconds - count
            print(time_left)
            self.value.set(time_left)


Shutdown_Timer = ShutdownTimerGUI()