title: Go: time.Format: how to understand meaning of '2006-01-02' layout?
body: <p>Given a time variable, I want to print year, month, and day.
  From the documentation, it seems that any layout can be used. For example, I don't see difference between layouts 2006-01-02, 2006-10-10, 1999-02-02.</p>
  
  <p>However, only layout 2006-01-02 returns what I expect. 
  Where can I find documentation on the meanings of '2006', '01', '02' in the layout?</p>
  
  <p>I played here with different layouts: <a href="https://play.golang.org/p/LihASulNoM" rel="noreferrer">go playground: testing layouts</a> </p>