My not OO version of timer for work and pause time (but it does use attributes for functions, which are objects, to avoid global declarations in functions).

from datetime import datetime, timedelta
try:
    import tkinter as tk
except ImportError:
    import Tkinter as tk


def whole_seconds(time):
    return str(time).split('.')[0]

def update():
    if not pause_toggle.paused:
        timer.set(whole_seconds(datetime.now() - start_time - pause_timer.pause_time))
    else:
        this_pause = datetime.now() - pause_toggle.pause_start
        pause_timer.set(whole_seconds(this_pause + pause_timer.pause_time))
    root.after(100,update)

def pause_toggle():
    if pause_toggle.paused:
        pause_button['text'] = 'Start pause'
        pause_timer.pause_time += datetime.now() - pause_toggle.pause_start
    else:
        pause_button['text'] = 'Start work'
        pause_toggle.pause_start = datetime.now()
    pause_toggle.paused = not pause_toggle.paused
pause_toggle.paused = False

def make_label(text):
    tk.Label(root, text=text, font=("Helvetica", 16)).pack()
    text_var = tk.StringVar()
    tk.Label(root, textvariable=text_var, font=("Helvetica", 16)).pack()
    return text_var
    
root = tk.Tk()
root.title('Work and pause time')
root.geometry('240x180')
root.wm_attributes("-topmost", 1)

timer, pause_timer = make_label("Work time"), make_label("Pause time")

pause_button = tk.Button(root, text='Start pause', font=("Helvetica", 12, "bold"), command=pause_toggle)
pause_button.pack(side=tk.BOTTOM, pady=10)
        
pause_timer.pause_time = timedelta(0)
pause_timer.set(whole_seconds(pause_timer.pause_time))
start_time = datetime.now()
update()

root.mainloop()