This copy of the Install OS X Yosemite application can’t be verified. It may have been corrupted or tampered with during downloading
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 & Time preferences pane, uncheck the box for “Set date & time automatically” & set the date back to just after the time Yosemite was initially released (October 16, 2014) then run the installer.
In the installation menu, choose Utilities / Terminal and type
1 | date 1020170014 |
then quit Terminal and the installer works.