Where are date_select_tag ?

Hi!
I want a date_select_tag inside a form_tag

But it does not work!

I looked for the docs at: http://api.rails.info/classes/ActionView/Helpers/DateHelper.html#M002208

And even, if I copy the code right inside - there are an error!

ActionView::Template::Error (can't convert Symbol into String) on line
#30 of /Users/kb1/code/rails/rateurdate/app/views/miniprofile/
meantimes/show_birthdate.fbml.erb:
27: <h1>t('miniprofile.meantimes.show_birthdate.headline'<br /

<span style="color:#98b8ce;"><%=

t('miniprofile.meantimes.show_postcode.subtitle') %></span></h1>
28: <% form_tag meantime_miniprofile_url(:type
=> :birthdate), :method => :post do |f| %>
29: <br><br><br>
30: <%= date_select("post", "written_on") %>
31: <br><br><br>
32: <br class="clear" />
33: <br>

    app/views/miniprofile/meantimes/show_birthdate.fbml.erb:30:in
`block in
_render_template__2824634172748094325_2184188140_3813724702546118430'
    app/views/miniprofile/meantimes/show_birthdate.fbml.erb:28:in
`_render_template__2824634172748094325_2184188140_3813724702546118430'
    app/controllers/miniprofile/meantimes_controller.rb:10:in `show'

This is the whole code:
    <% form_tag meantime_miniprofile_url(:type => :birthdate), :method
=> :post do |f| %>
    <br><br><br>
    <%= date_select("post", "written_on") %>
    <br><br><br>
    <br class="clear" />
    <br>
    <div class="buttons">
      <%= image_submit_tag "btn-weiter.png" %>
    </div>

    <% end -%>