Expiry of the Debug Certificate

The self-signed certificate used to sign your application in debug mode (the default on Eclipse/ADT and Ant builds) will have an expiration date of 365 days from its creation date.

When the certificate expires, you will get a build error. On Ant builds, the error looks like this:

debug:
[echo] Packaging bin/samples-debug.apk, and signing it with a debug key...
[exec] Debug Certificate expired on 8/4/08 3:43 PM

In Eclipse/ADT, you will see a similar error in the Android console.

To fix this problem, simply delete thedebug.keystorefile. The default storage location for AVDs is in~/.android/on OS X and Linux, inC:\Documents and Settings\<user>\.android\on Windows XP, and inC:\Users\<user>\.android\on Windows Vista and Windows 7.

The next time you build, the build tools will regenerate a new keystore and debug key.

转载自：http://developer.android.com/guide/publishing/app-signing.html