Thank you all for the replies!

I can't believe that it didn't cross my mind to change the date from the terminal ...

That worked fine and I was able to install the certificate!

However, I still can't get the email to work ... I'm really not sure what the problem is, I do read my email at home in TB, but I can't get it to work on the N900 ... it is eternally trying to access the account :-(

Oh well, I will probably end up fwding the emails to another account.
I have unsuccessfully tried to get the IT person to deal with this issue, so it may not be worth it anymore ...

Many thanks again for the help