timer button, timer runs when program starts:

from tkinter import *

x = 0
def run_counter(lab):
    def counting():
        global x
        x += 1
        lab.config( text = str(x) )
        lab.after(1000, counting)
    counting()
    
window = Tk()
window.title('Demo 02')
window.geometry("500x400+100+50")
lab0 = Label(window, width = 15, height=3,
             bg="#ccddee    ",
             text="cursor:\\nbottom_left_corner",
             justify="center",
             font="Arial 20 bold",
             cursor='bottom_left_corner')
lab0.pack()
run_counter(lab0)
btnExit = Button(
    window, width=15, height=2,
    text="exit",
    foreground="#ff2233",
    font="Arial 20 bold",
    command=window.destroy
)
btnExit.pack(pady=10)

window.mainloop()

Here's a modified version of the program that includes a "Start/Stop" button that toggles the timer on and off:

from tkinter import *

x = 0
timer_running = False

def start_timer():
    global timer_running
    timer_running = True
    run_counter()

def stop_timer():
    global timer_running
    timer_running = False

def run_counter():
    global x
    if timer_running:
        x += 1
        lab.config( text = str(x) )
        lab.after(1000, run_counter)

window = Tk()
window.title('Demo 02')
window.geometry("500x400+100+50")

lab0 = Label(window, width = 15, height=3,
             bg="#ccddee",
             text="cursor:\\nbottom_left_corner",
             justify="center",
             font="Arial 20 bold",
             cursor='bottom_left_corner')
lab0.pack()

lab = Label(window, width = 15, height=3,
             bg="#ccddee",
             text="0",
             justify="center",
             font="Arial 20 bold")
lab.pack()

btnStartStop = Button(
    window, width=15, height=2,
    text="Start",
    foreground="#33aa33",
    font="Arial 20 bold",
    command=lambda: start_timer() if not timer_running else stop_timer()
)
btnStartStop.pack(pady=10)

btnExit = Button(
    window, width=15, height=2,
    text="Exit",
    foreground="#ff2233",
    font="Arial 20 bold",
    command=window.destroy
)
btnExit.pack()

window.mainloop()

modify the program to change the text of the "Start/Stop" button to "Stop" when the timer is running. Here's the modified code:

from tkinter import *

x = 0
timer_running = False

def start_timer():
    global timer_running
    timer_running = True
    run_counter()
    btnStartStop.config(text="Stop", foreground="#aa3333", command=stop_timer)

def stop_timer():
    global timer_running
    timer_running = False
    btnStartStop.config(text="Start", foreground="#33aa33", command=start_timer)

def run_counter():
    global x
    if timer_running:
        x += 1
        lab.config( text = str(x) )
        lab.after(1000, run_counter)

window = Tk()
window.title('Demo 02')
window.geometry("500x400+100+50")

lab0 = Label(window, width = 15, height=3,
             bg="#ccddee",
             text="cursor:\\nbottom_left_corner",
             justify="center",
             font="Arial 20 bold",
             cursor='bottom_left_corner')
lab0.pack()

lab = Label(window, width = 15, height=3,
             bg="#ccddee",
             text="0",
             justify="center",
             font="Arial 20 bold")
lab.pack()

btnStartStop = Button(
    window, width=15, height=2,
    text="Start",
    foreground="#33aa33",
    font="Arial 20 bold",
    command=start_timer
)
btnStartStop.pack(pady=10)

btnExit = Button(
    window, width=15, height=2,
    text="Exit",
    foreground="#ff2233",
    font="Arial 20 bold",
    command=window.destroy
)
btnExit.pack()

window.mainloop()