使用线程可以几乎(不完全)与系统时钟同步:

Using thread can be almost (not completely) in sync with system clock:

from datetime import datetime
import threading
import time
import tkinter as tk

root = tk.Tk()

clock = tk.StringVar()
tk.Label(root, textvariable=clock, font="Times 20 bold", fg='light green', bg="dark green").pack()

def tick():
    while True:
        now = datetime.now()
        clock.set(now.strftime("%T"))
        time.sleep(1-now.microsecond/1000000)

threading.Thread(target=tick, daemon=True).start()
root.mainloop()