To clarify further, since March 31 seems to be year end for a lot of people...
 
AM-B opens the new year for posting in GL-O.  It closes out Income and Expense accounts to Retained Earnings, rolls the balances forward for GL-A and Financial Statements and advances the Fiscal Year date in AD-A.  It DOES NOT mean you have to be finished all posting and processing for last year.  Any further posting to last year will correctly post to 1 year past and if the posting happens to be to an income or expense account, the GL-O program will correctly make a closing entry to Retained Earnings as well.
 
When you want to prevent posting to last year (or any prior period) use AM-A to set the GL Close Date to the FIRST DAY of the period you want to enbable posting for.  So to prevent any transactions posting to a year ended March 31, 2004, set the close date to April 1, 2004.  If at some future date you need to make an adjustment back into last year, use AM-A to set the date back, make the adjustment and post it, then use AM-A to reset the date again.  This can be done as often as needed.