Cell automatically updates with current user

I want to have a cell the current user of a workbook automatically
Im using this code

Public Function UserName()
    UserName = Environ$("UserName")
End Function

And calling the function using =UserName()
The problem is the function doesn't automatically update when a new user opens the file. The new user has to recalculate the =UserName() function to display their name. What am i doing wrong?