You downloaded Yosemite or El Capitan Mac OS X and decided to install it a month or so later. As you start to install it and you get the error message:
“This copy of the Install OS X Yosemite(El Capitan) application can’t be verified. It may have been corrupted or tampered with during downloading.”

The certificate of this copy of the installation app has run out. I don’t know for how long the certificate is but there is a way out.

You set your system date/time to slightly later(one day or so) than the Modification Date/Time of the Installation app.
This way the app will ‘think’ that the certificate has not expired and let you install the new system. If the system is really too old then you can update it via Apple App Store after the installation.

1) Take a look at the Modification date of the Installation app (Install OS X El by right click on the application and selecting ‘Get Info’
2) Start you Utilities application called Terminal and use the following command to set the system date/time slightly later than the Modification Date/time. I used one day later and it worked fine:
sudo date {month}{day}{hour}{minute}{year}
Every bracket should be replaced with a two-digit number. For example, March 18th 2013 12:50 would become the following command:
sudo date 0318125013

Then restart the installation and it should not display this error any more.

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