The phrase "time killer" refers to an activity or object that wastes or uses up a lot of time without being very important or productive. It means doing something to pass the time, but it doesn't have much value or purpose.
A great time killer for those that love to build tracks:D. (levelsmack.com)
That'll be a good time killer for nights where I want to play a game but not get heavily invested in any of the more in - depth titles. (blog.us.playstation.com)