Many people get this error while installing something on their phone. Let me tell you how to solve it. I'm assuming that you are using a Nokia Symbian Phone.
Open Menu >Settings
In settings Find Date & Time
Set your Year to “2006″. Because some applications are certificated for 2005-2006. After setting your Year to 2006 install the application who gives certificate error.
Finish your installation and you can set your current date back:)