首先我们需要定义一个倒计时器的类，该类内需要包含倒计时器的状态（是否正在倒计时）和剩余时间。我们可以使用python的datetime库来实现时间的转换和计算操作。然后定义倒计时器的启动和暂停方法，以及设置倒计时值的方法。最后，我们可以通过使用Python Tkinter库创建窗口，并将倒计时器的状态、剩余时间和设置倒计时值的控件放置在窗口中。

代码示例：见下方

from tkinter import *
from datetime import datetime, timedelta

class CountdownTimer:
    def __init__(self):
        self.is_running = False
        self.time_left = 0
        self.start_time = None

    def start(self):
        if not self.is_running:
            self.is_running = True
            self.start_time = datetime.now()

    def pause(self):
        if self.is_running:
            self.is_running = False
            elapsed_time = datetime.now() - self.start_time
            self.time_left = self.time_left - elapsed_time.total_seconds()

    def set_time_left(self, minutes):
        self.time_left = timedelta(minutes=minutes)

    def get_time_left(self):
        return str(timedelta(seconds=self.time_left))

def update_timer():
    time_left = timer.get_time_left()
    time_label.config(text=time_left)
    if timer.is_running and timer.time_left > 0:
        timer.pause()
    window.after(1000, update_timer)

def start_timer():
    timer.start()

def pause_timer():
    timer.pause()

def set_timer():
    minutes = int(minutes_input.get())
    timer.set_time_left(minutes)

# 创建窗口
window = Tk()
window.title("可调倒计时器")
window.geometry("300x200")

# 创建控件
time_label = Label(window, text="")
time_label.pack()

minutes_label = Label(window, text="倒计时时间（分钟）：")
minutes_label.pack()

minutes_input = Entry(window)
minutes_input.pack()

start_button = Button(window, text="开始", command=start_timer)
start_button.pack()

pause_button = Button(window, text="暂停", command=pause_timer)
pause_button.pack()

set_button = Button(window, text="设置时间", command=set_timer)
set_button.pack()

# 初始化倒计时器
timer = CountdownTimer()

# 更新倒计时器的剩余时间
update_timer()

# 运行窗口
window.mainloop()