Calculate years and months from start and end dates

I have a rails employment management app Lets say a user creates a new work history entry. It contains a start and end date. How can i use ruby to calculate the number of months and years the user worked at that company? And also how to calculate the total years experience which is the sum of every work history.

eg work history 1
01-03-10 - 01-03-11
1 year

work history 2
01-03-11 - 01-03-12
1 year

total work experience 2 years

#form
<% form_for [@user, @history] do |f| %>
  <%= f.error_messages %>
  <p>
    <%= f.label :start %><br />
    <%= f.date_select :start %>
  </p>
  <p>
    <%= f.label :end %><br />
    <%= f.date_select :end %>
  </p>
   <p>
    <%= f.label :sector %><br />
    <%= collection_select(:history, :sector_id, Sector.find(:all), :id, :name) %>
   </p>
   <p>
    <%= f.label :company %><br />
    <%= f.text_field :company, :size => 70 %>
  </p>

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