title: Mongoid Date (DateTime) field not parsing correctly to store into database
body: <p>I have been having this issue for days and couldn't find any solution for this. It seems that I can't change the format of Date (&amp; DateTime) of a field in a Mongoid Document</p>
  
  <pre><code>class Project
    include Mongoid::Document
  
    field :deadline, :type =&gt; Date
  end
  </code></pre>
  
  <p>Then I can assign Date like this:</p>
  
  <pre><code>p = Project.new
  p.deadline = "20-10-2011"
  </code></pre>
  
  <p>But I can't assign in other formats:</p>
  
  <pre><code>p.deadline = "20/10/2011"
  ArgumentError: invalid date
      from /Users/pww/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/date.rb:956:in `new_by_frags'
      from /Users/pww/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/date.rb:1000:in `parse'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/extensions/date/conversions.rb:18:in `convert_to_time'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/extensions/time_conversions.rb:6:in `set'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/field.rb:109:in `set'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/attributes.rb:182:in `typed_value_for'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/attributes.rb:96:in `write_attribute'
      from /Users/pww/.rvm/gems/ree-1.8.7-2011.03@v3/gems/mongoid-2.0.2/lib/mongoid/fields.rb:161:in `deadline='
      from (irb):11
  </code></pre>
  
  <p>The thing is I tried changing the default format of Mongoid Date in several ways including </p>
  
  <pre><code>Date::DATE_FORMATS[:default] = "%d/%m/%Y"
  </code></pre>
  
  <p>which does work to display the data in that format but not to store the data in the format. I tried with localisation file as follow:</p>
  
  <pre><code>date:
      formats:
        default: "%d/%m/%Y"
        short: "%b %d"
        long: "%B %d %Y"
  </code></pre>
  
  <p>It doesn't work either. It's probably me not know how to get it right but it could be an issue with Mongoid. </p>
  
  <p>I am using:</p>
  
  <pre><code>Mongoid (2.0.2)
  Rails (3.0.6)
  ree (1.8.7-2011.03)
  </code></pre>
  
  <p>I am aware of this (https://github.com/mongoid/mongoid/issues/53) which is more a Date timezone issue. </p>
  
  <p>Any help and info with be greatly appreciated. </p>
  
  <p>Thanks.</p>