If you want it with a date that happens to be out of order (in the past) but still show up on the friends pages, post it in order with the current date, and then after it is posted, edit the date to be in the past.
LJ checks "Is this dated in order?" on post by simply looking to see if there are any dated-in-order entries with a time after the time you are currently posting. If there are none, it posts happily. If there are some, it throws an exception and tells you to backdate or date out of order, whichever language what you're posting with is using.
What you need to do if you're getting that error is a) make sure that you've got today's date on the entry you're posting right now, and b) go to your journal, look at the top of the page, notice that the entry up there is actually in the future, and EDIT THAT ENTRY to have either a) its correct date, or b) to be dated out of order.
This entry is brought to you by general confusion on the issue, and seeing one too many Support requests from People Who Don't Get It, and the requirement of polite language prohibiting us from strangling.