Expired Certificate is a error because the application that we installed in our Nokia N97 has expired, this error is not because the application is unsigned. To solve Expired Certificate you should not to change the date on your mobile, i give a better solution: Sign4ever.
How to use this application is very simple. Copy the application with expired certificate in the same folder that Sign4ever (in the PC), execute Sign4ever and follow the instructions of the blue screen (press any key to continue).
Created two files: a file .bak (backing of the application) and the implementation signed signed.sis. All applications signed are called signed.sis but you can change its name to the original.
Install the application signed.sis and the error of expired certificate is not a problem during the next 20 years.