代码：

import tkinter as tk
from time import time
 
print("hello world")
 
 
class TimeCounter:
    def __init__(self):
        root = tk.Tk()
        root.title("计时器")
        root.geometry('800x600')
        self.display = tk.Label(root, text="00:00", , font=('微软雅黑', 50), fg="red")
        self.display.pack()
 
        self.button_start = tk.Button(root, text='start', command=self.start)
        self.button_start.pack()
 
        self.Varmin = tk.StringVar()
        self.entrymin = tk.Entry(root, textvariable = self.Varmin)
        self.entrymin.pack()
 
        self.Varsec = tk.StringVar()
        self.entrysec = tk.Entry(root, textvariable = self.Varsec)
        self.entrysec.pack()
 
        self.paused = True
        root.mainloop()      # 进入消息循环
 
    def start(self):
        if self.paused:
            self.oldtime = time()
            self.paused = False
            self.run_timer()
    def gettime(self):
        try:
            min = self.entrymin.get()
            min = int(min)
        except:
            min = 0
 
        try:
            sec = self.entrysec.get()
            sec = int(sec)
        except:
            sec = 0
 
        self.minsec = 60*min+sec
 
        return self.minsec
 
    def run_timer(self):
 
        self.minsec = self.gettime()
        self.deltas = time() - self.oldtime   #  正向计时
        self.deltas1 = self.minsec - self.deltas
 
        print(self.deltas)
        print(self.deltas1)
        if self.deltas1>0:
            deltasstr = '{:.0f}:{:.3f}'.format(*divmod(self.deltas1,60))   #用 * 拆分这个元组
        else:
            deltasstr = '00:00'
 
        print(deltasstr)
        self.display.config(text = deltasstr) #更新 text
        self.display.after(1,self.run_timer)  # 间隔1毫秒再次执行run_timer函数，after循环定时器
 
 
TimeCounter()