Only? Only if it is rooted or on stock device.
Firstly I would say no you are wrong and second, it depends on the model maker.
On the 1st, Just download Good Lock from the app store, run quickstar from the menu. It is reasonably easy to follow  if not then check Youtube for Good Lock. It will do what you are after.
2nd. As for stock devices, adding am or pm to the status bar as never been on any Sony made device I have ever had. I can't speak for other brands.