Pull down the “Utilities” menu option and choose “Terminal”, then type the following command into the prompt: To determine if the Terminal date trick can fix those error messages and help you to successfully install MacOS X, you’ll need to turn to the command line while at the “Install Mac OS X” boot menu. If you’re on a computer without internet access, or if you encounter the problem during an alternative OS X installation method and thus can’t access System Preferences, turning to the Terminal to set the date is the next option. 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.

Install os x el capitan terminal