You could reverse the numbering system so it would appear to be counting up instead of down? Or you could create a text block that changes contents every second but that would have to involve a lot of actions to keep the development within Lectora. I think th easiest way is for something to be coded in (div.time maybe?)
I do think Tim’s suggestion building a custom timer is the best way to go.
You could create a timed action that countsup a variable at every second or any other interval. That would stop when going to the next page. The next page you however either need another variable, because i presume you want to addup all time spent on pages on the end of the course and get a total. If thats the case you might want to check the build in timers…they run over subsequent pages and thus can hold your total time.