Now install your new theme. If you still see the Expired certificate error, change the year back another year and try installing again.
When it's installed, follow the instructions again, setting the year back to the present by highlighting the year and pressing the Up navigation key once.
Was able to finally get Skype to sign in (perhaps a coincidence?)
Turn off phone.
Remove battery, leave out for at least a minute.
Reinstall battery, reboot phone.
Wait for at least 10 minutes before running Skype.
Try running Skype (will require setting a few preferences, and then entering name/password).
Pray to the god of your choice.
Worked with N79, might also work with N78, E5 and other s60 phones?