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first is you need a code to get the date and save it to variable. Try to watch this: How to create a Digital Clock | GDevelop 5 - YouTube

Then this is how you can calculate how long they are being offline. How To Calculate Duration Between Any Two Dates Using Paper/Pen || Duration Calculation || Tutorial - YouTube

Then just convert them to a minutes or seconds. My fish grow every 3 mins if the result in the computation is 1hr and 30mins just convert the 1hr to 60mins then add the 30 mins then the result is 90mins. then i just divide the 90 to 3 because every 3 mins my fish got additional 1 age. 90 / 3 = 30 then im gonna add the 30 to the age of my fish. the same to the health of my fish and for the coins. 

I Hope you understand my explanation. Because I'm really bad in English. XD