This copy of the Install OS X Yosemite application can’t be verified. It may have been corrupted or tampered with during downloading
Your installation file for Yosemite has had its digital certificate expire. Which means after a certain date it will no longer work. To work around this issue you open the Date &amp; Time preferences pane, uncheck the box for "Set date &amp; time automatically" &amp; set the date back to just after the time Yosemite was initially released (October 16, 2014) then run the installer.