The Android SDK generates a "debug" signing certificate for you in a keystore called debug.keystore.The Eclipse plug-in uses this certificate to sign each application build that is generated.
Unfortunately a debug certificate is only valid for 365 days. To generate a new one, you must delete the existing debug.keystore file. Its location is platform dependent - you can find it in Preferences ->Android ->Build -> *Default debug keystore.
If you are using Windows, follow the steps below.
DOS: del c:\user\dad.android\debug.keystore
Eclipse: In Project, Clean the project. Close Eclipse. Re-open Eclipse.
Eclipse: Start the Emulator. Remove the Application from the emulator.
If you are using Linux or Mac, follow the steps below.
Manually delete debug.keystore from the .android folder.
You can find the .android folder like this: home/username/.android
Note: the default .android file will be hidden.
So click on the places menu. Under select home folder. Under click on view, under click show hidden files and then the .android folder will be visible.
Delete debug.keystore from the .android folder.
Then clean your project. Now Android will generate a new .android folder file.
