If you run into either error message while trying to update or install 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.