Energy or lives regen by time

"When you are out of the app"
I believe the expected way to do this involves measuring the time elapsed when you start the application. In other words, record the time you last quit the application. Read that value when the application starts. Use System.DateAndTime math to find the real elapsed time and use it as desired.

For a micro-transaction game or game where points and leaderboards matter, this will not suffice, since people can change their date-and-time settings on their computer to fool the game. In such cases, these calculations are done server-side (on your company's server).