text: In my case, this command returned the following:
  See that date and time hiding in the middle of that Terminal output? They show the last time my malware definitions were updated, in this case on May 26. To force my Mac to update its malware definitions, I launched System Preferences > Security > General, and then unchecked and re-checked the box in front of "Automatically update safe downloads list."
  Now after pasting that command into Terminal again and pressing Return, I see that the latest malware definition was sent today:
  After the last security update from Apple, the system is supposed to check daily for updates to the OS X malware definitions. This little tip insures that you ultra-paranoid types can update more often than that.
  Update: Adam Christianson of The MacCast wrote up a nice little Mac app to check the version and date (in your local time, not GMT) of the latest malware definition update on your machine. You'll still have to force the update using the method described, but this app keeps you from having to launch Terminal. The app is available here (download link), courtesy of The Mac Observer.