I think you can obtain system date/time through a plugin or simple expressions (i'm not sure about this last).
First, set the date and time in a persistent data like local storage everytime the app's closed. Then when the app is opened again, calculate the hours/days difference between current and stored time.
Sorry about my english D:
You have been very helpful.
The next question is, how to retrieve the date time in an easy to calculate way that allows me to easily calculate how many 24hours have passed since last launch ?