When I installed GForge recently, I had to generate an SSL certificate as part of the installation since GForge runs on https. However, the make-ssl-cert tool does not provide a way to customize the validity of the generated certificate, which is 30 days by default.
As it turns out, this problem is known for 3 years, and it is still not fixed in Ubuntu 8.04.
The easiest workaround is to edit the make-ssl-cert script using
whereis make-ssl-cert sudo nano [path-to/]make-ssl-cert
and replace the line