2008/01/01 January
    Expenses:Books          $10.00
    Assets:Cash

2008/01/31=2008/01/01 End of January
    Expenses:Books          $10.00  ; [=2008/02/01]
    Assets:Cash

2008/02/01 February
    Expenses:Books          $20.00
    Assets:Cash

test reg --aux-date
08-Jan-01 January               Expenses:Books               $10.00       $10.00
                                Assets:Cash                 $-10.00            0
08-Feb-01 End of January        Expenses:Books               $10.00       $10.00
08-Jan-01 End of January        Assets:Cash                 $-10.00            0
08-Feb-01 February              Expenses:Books               $20.00       $20.00
                                Assets:Cash                 $-20.00            0
end test