http://www.livejournal.com/support/faqbrowse.bml?faqid=33
Why are my journal entries showing an incorrect date/time? Why can't I use 12-hour time?
If the date or time of an entry is showing up incorrectly, this most likely means that the clock on your computer is now, or was at some time, set to the wrong date or time. This could affect all posts made after the incorrect post. First, make sure that the date, time, and time zone are all now set correctly on your computer. Then edit the date and time of the entries that are incorrect.
This can also occur if you have used the web client to update your journal from a web browser in which JavaScript is not enabled. This will cause the web client to fill in the form with the current time according to LiveJournal's servers rather than with your local time. In this case, you should simply edit the entries on which the time is incorrect and enter the correct time.
24 HOUR TIME
If you are manually entering the time of an entry, please note that the Update Journal page uses a 24-hour clock and that LiveJournal does not provide a way to change the clock to 12-hour time. This means that if you enter "8:00", LiveJournal will interpret this as 8 AM; in order to have 8 PM instead, you will need to enter "20:00".
Many clients available for download include the ability to use 12-hour time. For more information on your preferred client, consult its documentation or its menu options.
FURTHER READING
How do I edit or delete a journal entry I wrote?
http://www.livejournal.com/support/faqbrowse.bml?faqid=2
Why does LiveJournal display incorrectly in my browser?
http://www.livejournal.com/support/faqbrowse.bml?faqid=210
What is a downloadable client? Where can I get one?
http://www.livejournal.com/support/faqbrowse.bml?faqid=158