title: Problem with updating timer in an animation in flutter
body: <p>I am trying to make a countdown timer using flutter, but the problem is, my timer seemingly does not update, I am unable to understand what could be the issue.
  <a href="https://dartpad.dev/90bf084d626c2d78f683f2f235155ec7" rel="nofollow noreferrer">dartPad link to run the code.</a></p>
  <p>Also I would like to know how to run the timer in reverse, that is starting from 0 seconds.</p>