Can't open the application Calendar. -1712

OS 10.8.4, Macbook Pro, 2.5 GHz i5

Calendar v 6.0 is in applications folder and is 40.8 MB. It was opened 8 days ago and had been open for several days. I think I last used it 3 days ago.

The iCal alias was in the dock and showed open (triangle). Command i showed the linked application as Calendar in the application folder. I clicked on the alias in the dock yet nothing happened. I double clicked the Calendar in the Application folder and nothing happened. When I checked all the windows open there was no Calendar window.

The next time I tried to open Calendar from the iCal alias in the Dock or the Calendar in the Applications folder I got the pop up notification with the Finder faces "The application 'Calendar' can't be opened. -1712"

Thanks for your assistance.