Slide Timer

I used a slightly different approach than Mark's to achieve what you are looking for. Specifically, I used on "off the slide 1 second object animation" that works off the same concept as the 1 second slide layer timeline. I put this on the master slide along with some standard triggers that go with it. The triggers handle how often the animation repeats and handles things like an even start time in minutes as well as when to subtract a value from minutes as well as from seconds, etc. Spend some time reviewing these and ask if you need any clarification.

On the slides, there are some other important triggers. On each slide, you can customize the time for the slide by adjusting the "Set Time_S (Seconds) and "Set Time_M (Minutes) variables. Be sure to change the values as needed.

The total time variable that I use elsewhere will auto calculate based on the other values; do not change these.

The variable triggers are the same you had before and handle the state changes to manage the display for single digit numbers. 

I also simplified your variables to use 1 global variable for minutes and 1 for seconds. Because this is a standard variable, and because the animation that adjusts it is on the slide master, the adjustments will continue on the main slide even while a light box slide is displayed.  

See attached and reach out with any questions.