Joakim, did Ruben's post get things working for you? If so, do not forget to come back and use the Accept as Solution button on his post to give him credit and let others know the suggestion worked. If you are still working upon things, leave an update for us.
Here is a Design Blog link with some further info regarding date functions too:
To help users find verified answers, please do not forget to use the "Accept as Solution" button on any post(s) that helped you resolve your problem or question. I now work a compressed schedule, Tuesday, Wednesday and Thursday, so those will be the days I will reply to any follow-up posts.