Ich habe versucht einen timer zu machen, jedoch funktioniert dieser nicht recht.

from tkinter import *
from tkinter.ttk import *
from time import sleep


root_timer = Tk()


timer = 10

lbl_timer = Label(root_timer, font=("Times New Roman", 26, "bold"),
                                    background="white",
                                    foreground="black")

lbl_timer.config(text=str(timer))

lbl_timer.pack(anchor="center")

def timer_up_press():
    global timer
    timer += 1
    lbl_timer.config(text=str(timer))

    lbl_timer.pack(anchor="center")

def timer_down_press():
    global timer
    if timer == 0:
        print()

    else:
        timer -= 1
        lbl_timer.config(text=str(timer))

        lbl_timer.pack(anchor="center")

def timer_start_press():
    global timer
    while True:
        if timer == 0:
            lbl_timer.config(text='Time is Up')
            sleep(1)

        else:
            timer -= 1
            lbl_timer.config(text=str(timer))
            sleep(1)

button_timer_up = Button(root_timer, text='Up', command=timer_up_press)
button_timer_down = Button(root_timer, text='Down', command=timer_down_press)
button_timer_start = Button(root_timer, text='Start', command=timer_start_press)

button_timer_up.pack()
button_timer_down.pack()
button_timer_start.pack()

root_timer.mainloop()