ORA-01830: date format picture ends before converting entire input string / Select sum where date query

I think you should not rely on the implicit conversion. It is a bad practice.

Instead you should try like this:

datenum >= to_date('11/26/2013','mm/dd/yyyy')

or like

datenum >= date '2013-09-01'

You can try this:

Select To_date ('15/2/2007 00:00:00', 'DD/MM/YYYY HH24:MI:SS'),
       To_date ('28/2/2007 10:12', 'DD/MM/YYYY HH24:MI:SS')
  From DUAL;

Source: http://notsyncing.org/2008/02/manipulando-fechas-con-horas-en-plsql-y-sql/