Saving dates

Stewart
2006-12-14T03:57:51Z

Hi having trouble saving dates in my form i have the following in the
view

<%= date_select("details", "open_date") %>

and i have

@exhibition.open_date = params[:details][:open_date]

and then

@exhibition.save


All the other values form (not seen here) get saved but not the dates.
What am i doing wrong ?