A Common Event will continue to run, even if you change Map. My Farm system has Carrots growing whilst I'm away; it can be made to work.
The best recommendation I can make is to create a mini-Project, as a test-bed for this. Write out on paper, first, a few Switches and Variables that you'll be using, and write out (still on paper...) a 'pseudo code' that makes sense to you. Then (and only then...) create the Switches and Variables in the database, get it up and running, even modestly, on your Map, then create a second Map and go to it, see what happens. There's nothing to beat experience in issues of this sort. We can help you with detail points and suggestions, but in the end, you have to master it (progressively...) yourself if you're to manipulate it, even more so if the whole Game turns on such an axis.
Do the first steps, and see where (if..?) you get stuck; we'll guide you on from there.
As I said above there are many ways to do this (including taking on a 'ready-made' plug-in..!), but beating your own sword is always satisfying.
Do you need to display the time, for instance, or simply have Tints reacting to the passage of time..?
Get a simple counter running first, before diving in to all the potential complications.
