I would associate a non-persistent entity (e.g. Timer) to a Session and capture the Start (DateTime) there. You make use of a snippet which you place on all the applicable pages, or put that in the Layout you use for all these pages (for example Atlas_Topbar).
In this situation, the same Timer object is shown on all your pages.
In the snippet: put a listview retrieving the Timer(s) over association from Session and set page size to 1. Create an object somewhere in your navigation flows, such that 1 Timer is created if it doesn't exist yet.
You can play around with buttons:
If needed you can make use of a MicroflowTimer or MicroflowRepeater widget to recalculate the number of seconds/minutes past and show that. Preferably use nanoflows to minimize the load on the server.