Hmm. Jul 17 is the default icon for the calendar app (even on El Capitan, which I'm running); maybe there's a preferences setting as to whether it displays the current date in the dock or not?
Standard warning: modern versions of OS X consume a lot more RAM, so you may find that you'll need an upgrade to avoid swapping. Also the modern Photos app is a lot more annoying than the old iPhoto; that "upgrade" was kind of a bummer. But otherwise El Capitan will run just fine on a 2009 Mini.
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I disagree.

El Capitan does run fluidly on a 2009 Mini with 4GB of RAM,
if you don't run any memory-intensive apps. Most likely, you'll want more than 4GB, though.
But El Capitan doesn't run any slower (or faster) on an HDD than does any other version of OS X. Therefore, an SSD will
not improve El Capitan any more than it would improve any other version of OS X. If you can't live without an SSD, don't blame the OS for that fact.