This is interesting as I’ve just tried the same thing with one of the notes that I have in Apple Notes and the time stamp is correct.
Is the time zone set correctly in your Runbox account?
For anyone reading this who is not familiar with Apple Notes (and who might like to take advantage of the app), this isn’t a Runbox feature.
Apple Notes works via IMAP and stores notes as messages in a folder called Notes in your email account. You don’t see the Notes folder in Apple Mail because it knows it is being used by Apple Notes and so it hides it. However, you can read notes in other email apps and in the webmail interface as they appear as messages. It is quite a convenient feature