Modifying code for Classroom clock by By jdeboi

Hi, I made a clock I saw on inscrutables by jbedi, link below:

This is my first arduino project, I am making this for my classroom so I can have one on one chats with my students without being disturbed by the others, while keeping track of the time.

The original clock has a countdown function that shows up after the end of each period, I need to modify the code so the countdown starts with a press of a bottom instead, I have tried my best to make it work but with no success.

code:

Thank you

Gary