How to 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.

Be aware that the given period in after method is not guaranteed to call the function after the precise period, because it could be delayed if the application is busy, resulting from the fact that Tkinter is single-threaded.