How can I make a button clickable if countdown har reached 0?

I have a countdown until marvel movies have their premier and would like to have a button "Book Tickets" that is unclickable when counting down, and then make it clickable when the countdown has reached 0. How do I do that?

I have tried to make if-statements, but something is wrong.

var clock = $('.clock').FlipClock(new Date("July 5, 2019 00:00:00"),{
  clockFace: 'DailyCounter',
  countdown: true
  });

You can see the countdown I currently have on: http://www.student.city.ac.uk/~aczc972/