title: In what layer of a project do common extension methods belong?
body: <p>I'm curious what "layer" of a system common extensions that are global to the application belong in.  For instance, I may have extensions that let me use the Rails-like "DaysAgo", "MonthsAgo", etc. type of extension method  on integers.  What layer of a project does this typically belong in?  I was thinking Infrastructure, but that seems to mean database-related (e.g. base repositories and data access).  I have a "Core" library project, so maybe it belongs there?</p>
  
  <p>I understand that you want to group extensions that are related to a specific group of classes, but these are essentially used across the entire application.  In the days before extension methods, they would be in a <code>Utilities</code> static class or the like, so where should they live now?</p>