1 hour countdown timer

I am creating a "technology-enhanced" escape room/scavenger hunt. Users will have one hour to complete the scenarios - solving riddles, clues, codes and collecting 10 historical objects that must be placed in the correct order to complete their mission. I've got everything working. Now I need to have a timer that starts when they solve the first clue.  The timer needs to work across the entire project - not just for a single graded interaction.  Right now we're thinking 1 hour, but we may need to make it 90 minutes.

I'd like for them to be able to click a top menu button to access the timer at any time. Thought I could have the timer on a separate slide with layers and variables, but that's as far as I can figure this one out. I'm coming back to Storyline after a brief hiatus and am having to figure things out again.

Any help would be greatly appreciated!!