If there are just 20 posts, then I would copy and paste them into new posts. Check the box at the bottom of the posting page that says "Edit timestamp" and set them for whatever date in the past that you want.

See I tried that before, but what I didnt realise till now was that I had to have the little box checked beside the 'edit timestamp'

Yes I am a dumbass.. thanks for your understanding and non patronising reply to this daft question!!

Don't feel bad. I do that all the time. Then, when I see that my post doesn't have the right date and time, I go back in, check the box and submit. Then I remember that I didin't change the time the second time, so I go back in . . . etc.