Python: Microwave Timer

I am at a very beginner level with Python and I have a simple project for myself. My goal is to build a simple timer with only two functions using tkinter and python. The first button starts the countdown timer at 30 seconds. And the second button stops it. However, I would like each additional press of the first button to add an additional 30 seconds to whatever time is currently left on the countdown clock. The first two elements have proven not too difficult using this prior thread, but adding additional time to the active timer is something I cannot reach.

Essentially, the goal is to recreate the functionality of the “+30 seconds” button on a microwave, with an updating display, where the initial press both adds 30 seconds and starts the timer, each subsequent press adds an additional 30 seconds to the countdown timer, and the second button stops the timer.

Here’s what I’ve been using for the basic timer logic.

def countdown(t=30):
while t:
    mins, secs = divmod(t, 60)
    timer = '{:02d}:{:02d}'.format(mins, secs)
    print(timer, end="r")
    time.sleep(1)
    t -= 1
print('Done!')