That method requires the Mac to have active internet access, however. To do this, go to the Apple menu > System Preferences > Date & Time, and be sure the “Set date and time automatically” option is checked: If you run into either error message while trying to update or install Mac OS X while from an active OS X boot (say, a standard upgrade from the App Store), you can typically resolve the problem simply by setting the Date & Time on the Mac to be determined automatically.
It’s possible to encounter these errors during basically any type of installation attempt in OS X El Capitan or OS X Yosemite, ranging from the standard App Store update, using Internet Recovery, to clean installs, and using a bootable installer volume on a target Mac.
