Hi Andrew,
This is difficult to answer without knowing your setup. Can you first confirm the time zone your computer is set to, this might have something to do with that. Do you have access to another Windows Computer that you can see if you have the same issue in Report Designer?
If the first suggestion above doesn't help with this error, or you don't have access to another computer, would you be able turn on Method Support on your account so we could have a look? This can be done by going to Customize->Users, then go to the "Edit an Exisiting User" Section, and make sure there is a checkmark in the Active Column for MethodSupport. I would also need to know your Company Account Name that you use when you login, you can send me an email with this information, either through the Forums or in the address in my signature. Please include the time zone your computer is set to in the email, for testing purposes.
-Ben