What is the "Backdate Entry" option and how do I use it?
Setting the "Backdate Entry" option, available for each journal entry in an individual journal, ensures that the entry will appear only on pages where its placement is based on the date and time you specify, specifically the Recent, Calendar, and Day views, but not the Friends view. You cannot backdate entries to communities, because a community is usually a forum for recent news or events in which backdating is unnecessary.
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 an entry with a future date or time. If you give an entry 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 InsaneJournal will not accept the entry.
On someone's Friends page, the placement of your journal entries is based on when you actually posted them, not on the dates and times you entered. Checking the "Backdate Entry" box prevents your entry from appearing on those pages.