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 the debug.keystore file. The default storage location for AVDs is in ~/.android/ on OS X and
Linux, in C:\Documents and Settings\<user>\.android\ on Windows XP, and in C:\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.
轉載自：http://developer.android.com/guide/publishing/app-signing.html