iPhone prototype app using timer at the background -Swift-

I am a software developer on expertise other platforms / programming languages. I want to develop an iPhone application using background Timer. I am looking for someone who can show me how to create a simple iPhone app using Timer at the background so I can schedule small tasks regularly. I expect you to provide me instructions to create a new single view application and use a Timer for background tasks. Please note when I try a basic youtube video I came across an error "Can't end BackgroundTask: no background task exists with identifier" so if you come across this issue and know how to resolve it, it's a plus. Please note Timer should be triggered even though application is at the background.