The time refers to the date that it was released, and as such, you will only see mid-2010, mid-2011 or late-2012. It comes in handy to distinguish some hardware was updated twice in a year.
If you bought a Mac Mini in February of 2011, it is a mid-2010. If you bought it in December of 2011, it's a mid-2011.
You might as well use the "Ultimate Mac Lookup" on
Everymac.com if you're in doubt.