Where is the Rails method that converts data from `datetime_select` into a DateTime object?

When I use <%= f.datetime_select :somedate %> in a form, it generates HTML like:

<select id="some_date_1i" name="somedate1(1i)">  #year
<select id="some_date_2i" name="somedate1(2i)">  #month
<select id="some_date_3i" name="somedate1(3i)">  #day
<select id="some_date_4i" name="somedate1(4i)">  #hour
<select id="some_date_5i" name="somedate1(5i)">  #minute

When that form is submitted, the somedate1(<n>i) values are received:

{"date1(1i)"=>"2011", "date1(2i)"=>"2", "date1(3i)"=>"21", "date1(4i)"=>"19", "date1(5i)"=>"25"}

How can I convert that into a DateTime object?

I could write my own method to do this, but since Rails already is able to do the conversion, I was wondering if I could call that Rails method to do it for me?

I don't know where to look for that method.

I'm ultimately trying to solve "How to handle date/times in POST parameters?" and this question is the first step in trying to find a solution to that other problem.