import tkinter as tk

***** VARIABLES *****

use a boolean variable to help control state of time (running or not running)

running = False

time variables initially set to 0

hours, minutes, seconds = 0, 0, 0

***** NOTES ON GLOBAL *****

global will be used to modify variables outside functions

another option would be to use a class and subclass Frame

***** FUNCTIONS *****

start, pause, and reset functions will be called when the buttons are clicked

start function

def start():
global running
if not running:
update()
running = True

pause function

def pause():
global running
if running:
# cancel updating of time using after_cancel()
stopwatch_label.after_cancel(update_time)
running = False

reset function

def reset():
global running
if running:
# cancel updating of time using after_cancel()
stopwatch_label.after_cancel(update_time)
running = False
# set variables back to zero
global hours, minutes, seconds
hours, minutes, seconds = 0, 0, 0
# set label back to zero
stopwatch_label.config(text='00:00:00')

update stopwatch function

def update():
# update seconds with (addition) compound assignment operator
global hours, minutes, seconds
seconds += 1
if seconds == 60:
minutes += 1
seconds = 0
if minutes == 60:
hours += 1
minutes = 0
# format time to include leading zeros
hours_string = f'{hours}' if hours > 9 else f'0{hours}'
minutes_string = f'{minutes}' if minutes > 9 else f'0{minutes}'
seconds_string = f'{seconds}' if seconds > 9 else f'0{seconds}'
# update timer label after 1000 ms (1 second)
stopwatch_label.config(text=hours_string + ':' + minutes_string + ':' + seconds_string)
# after each second (1000 milliseconds), call update function
# use update_time variable to cancel or pause the time using after_cancel
global update_time
update_time = stopwatch_label.after(60, update)

***** WIDGETS *****

create main window

root = tk.Tk()
root.geometry('485x255')
root.title('Stopwatch')

label to display time

stopwatch_label = tk.Label(text='00:00:00', font=('Arial', 80))
stopwatch_label.pack()

start, pause, reset, quit buttons

start_button = tk.Button(text='start', height=5, width=7, font=('Arial', 20), command=start)
start_button.pack(side=tk.LEFT)
pause_button = tk.Button(text='pause', height=5, width=7, font=('Arial', 20), command=pause)
pause_button.pack(side=tk.LEFT)
reset_button = tk.Button(text='reset', height=5, width=7, font=('Arial', 20), command=reset)
reset_button.pack(side=tk.LEFT)
quit_button = tk.Button(text='quit', height=5, width=7, font=('Arial', 20), command=root.quit)
quit_button.pack(side=tk.LEFT)

***** MAINLOOP *****

run app

root.mainloop()