You need to save the data to a save game object, that can then be saved when exiting, then loaded when opening. You should read through Epic’s documentation on this:
Also, it’s wasteful to utilize the tick function for this type of functionality. I don’t think this needs to run every frame, so you should also look into utilizing Timers and having it run once or twice a second (depends on how incremental the change is in time and the sun rotation).