When trying to place an order online, I get a security message saying 'The security certificate has expired or is not yet valid'. Is it still safe for me to proceed?

A security certificate contains data that is transferred from one computer to another to prove the authenticity or security of information on the internet. You're probably receiving this message because your computer's date and time are set incorrectly. 

If this isn't the case and it is one particular site that you're having problems with, get in touch with them to make sure that the problem isn't with the website itself. 

When you're making a payment online also remember to check that the web address begins with https:// rather than just http:// and look for the secure padlock icon alongside the address.