If you want to perform a fresh installation of OSX 10.10 Yosemite on an older MacBook(Pro) you can obtain the installer with AppStore and then create the USB installation media.

In case you encounter an Installer Verification Error that prevents you from installing the new system try the following. Its not the system date not bad download as internets says.

This may also help you when encountering problems (i.e. USB Mass Storage) after system upgrade to Yosemite.

Try these boot keyboard shortcuts until you hear boot sound two times:

• Cmd+Alt+R+P when powering on the Mac (NVRAM Reset)
• Alt+Ctrl+Shift+PowerON alltogether (SMC Reset)

No problem, you are wolcome :-)