Storing dates from a date_select does not work

Hi there
I’m having problems storing dates in a DB(postgreSQL)
I have the field dtmfechanac of type date

I have this in the view
<%= date_select ‘usuario’, ‘dtmfechanac’, :start_year => 1900 %>
and I have this in te controller
data = {
“dtmfechanac” => params[:usuario][:dtmfechanac]
“dtmfechainicio” => params[:usuario][:dtmfechainicio]
}

But it doesnt work I have tried
params[:usuario][:dtmfechanac].to_s .to_time but still no luck
I get this error

You have a nil object when you didn’t expect it!
The error occured while evaluating nil.to_formatted_s

but the date is being sent I see this in the same error page

Parameters: {“commit”=>“Ingresar Tomador”,
“usuario”=>{“strtel1”=>“lkh”, “strteltrabajo”=>“kj”,
“strtel2”=>“jlkj”, “strtel3”=>“hl”, “strtramite”=>“1”,
“dtmfechanac(1i)”=>“2006”, “dtmfechanac(2i)”=>“6”, “strnombre”=>“kjh”,
“inttipoentierro”=>“3”, “strdirtrabajo”=>"", “intcodentidad”=>“1”,
“dtmfechanac(3i)”=>“29”, “intcodciudad”=>“1”, “strocupacion”=>"",
“dtmfechainicio(1i)”=>“2006”, “inttipoid”=>“1”,
“dtmfechainicio(2i)”=>“6”, “strdircasa”=>"",
“dtmfechainicio(3i)”=>“29”, “controlinmpresos”=>"",
“intcodparentezco”=>“1”, “controlreaseguros”=>"", “intid”=>“897”,
“intestado”=>“1”, “intedad”=>""}}

as you can see the dtmfechanac and dtmfechainicio and NOT nil

I dont know what can be the problem

thanks in advance