Use a Timer in Tkinter

Tkinter root window has a dedicated method after that calls a function after a given time period -

after(ms, func)

ms is the interval in the unit of ms,

func is the called function name.

try:
    import Tkinter as tk
except:
    import tkinter as tk
    
import time

class Clock():
    def __init__(self):
        self.root = tk.Tk()
        self.label = tk.Label(text="", font=('Helvetica', 48), fg='red')
        self.label.pack()
        self.update_clock()
        self.root.mainloop()

    def update_clock(self):
        now = time.strftime("%H:%M:%S")
        self.label.configure(text=now)
        self.root.after(1000, self.update_clock)

app=Clock()

self.root.after(1000, self.update_clock) calls the function itself after 1000 ms, therefore, update_clock() function is executed at the interval of 1000 ms and displays the current time in the Tkinter label.