:order - Set to an array containing
:day, :month and :year to customize
the order in which the select fields are shown. If you leave out any of the
symbols, the respective select will not be shown (like when you set
:discard_xxx => true. Defaults to the order defined in the
respective locale (e.g. [:year, :month, :day] in the en locale that ships
with Rails)
:date_separator - Specifies a string to separate the date
fields. Default is “” (i.e. nothing).