ISO 8601 date format for microformats
strftime('%Y-%m-%dT%H:%M:%S%z');
If you're in Ruby, you can access this the quick way with:
$ irb >> require 'time' => true >> Time.now.iso8601 => "2008-07-09T16:13:30+12:00"
Rails example:
<abbr class="dtstart" title="<%= event.start_date.iso8601"><%= event.start_date.to_s(:long) %></abbr>