This bug is observed in one of our installation and can be replicated on demo.idempiere.com.
A quick check: DateAcct appears on these "Line" tables: GL_JournalLine, C_TaxDeclarationLine, and C_BankStatementLine.
Currently, DateAcct in hidden in window GL Journal and is only displayed when currency other than the base currency is selected in the GL Journal tab. I think it is for currency conversion calculation.
However, currently there is no validation to ensure that the newly entered date is within the period specified in GL Journal tab.
For example: I selected period Feb 13 in GL Journal tab, and i can entry 18 Oct 2012 for DateAcct in the GL Journal Line. The document can be completed and the journal will be created with DateAcct = 18 Oct 2012 and Period Feb 13!