I am sorry to hear that you missed your cruise. I know it must have been a big disappointment however it is really up to you to correlate the dates with the "days" of your cruise.
It may have been a miscommunication with a Royal Caribbean agent that resulted in confusion about the days of the cruise but you really had plenty of time to realize what "day" the cruise left. You also booked the cruise online and the days are on the initial booking page. A simple look at a calendar and the itinerary, in your documents, would have been sufficient for you to realize what day the cruise departed. If the date was wrong, in your documents and SeaPass, then you deserve a full refund but I can't believe it was.
I understand your disappointment but I agree with Royal Caribbean, on this issue, that you are not due a refund of your cruise fare. If you are receiving a refund of taxes and port fees I think they are being more than fair.
Take care and I hope you make it to your next cruise.