cheerful...ensures that the entry will appear only on pages where its placement is based on the date and time you specify.
Is it possible to specify exactly which views these are? To my understanding, it appears on the calendar/day view and the lastn view, but not the friends view.
You cannot backdate entries to communities.
It would be kind of nice to explain why this is. I assume there's a reason, and if it's not too technical, a very short explanation would make the statement less abrupt.
The date and time of a backdated entry does not need to be in the past. If you post an entry dated in the future, you will need to use the "Backdate Entry" option. This will allow you to continue posting with a normal date and time. Otherwise, LiveJournal will not accept your entry.
Edit:
kyrielle pointed out that in my rewording I got the concept wrong. I've updated my wording to be consistent with reality. :)The date and time of a backdated entry do not have to be in the past. You should also use this option if you would like to make a post with a future date or time. If you give a post a future date or time without selecting the 'Backdate' option, and subsequently try to make a post dated earlier than your future-dated post, you will encounter an error and LiveJournal will not accept the post.
...and can later be modified when you edit the entry (http://www.livejournal.com/support/faqbrowse.bml?faqid=2).
...and can be modified later by editing...
When you use this option, your entry is not accessible by paging back through certain views...
This will probably be redundant if the information is added at the top about where, specifically, the option causes the entry to appear. Perhaps the rest of that sentence (but you can always find an entry through the Calendar view (http://www.livejournal.com/support/faqbrowse.bml?faqid=70) ) could also be moved up there if that adjustment works.