I think you would have to gather the difference in time between plays. Something like save the current time/date and the next time the user open the app compare the current time to it. Then convert it to seconds and do the calculations on how much their score should have improved in that amount of time. This of course lets the user cheat if they change the time on their device.
that actually sounds like a pretty plausible way of doing it thanks!
I'm still a bit of a novice when it comes to Construct though - could you advise me as to how I can a) save the time/date specifically (instead of the whole game) b) compare the two times? I've read the manual cover to cover a while ago but I can't remember anything in there that would help me with this!
Oh just one other little note - the game is mobile based.
Thanks for your help!