Hi,
I don’t think that doing anything when the app is not running is possible through Blueprints. A possible solution I can imagine is that you save the state of the timer when the player closes the app to a database, and when he opens the app again, you query the database. If you don’t want to go online, you could save the timer state along with the other variables you save about the player.
[EDIT]
Reading through your question again, what I provided is not eaxactly what you want to do, sorry. However it can still work if you save the actual date and time and then with some math magic you calculate the time passed since logout, so you can also calculate how many life the player should have received in that time period.