DateTime conditional for RSS parser

I've been struggling with an adjustment to a rss parsesr (a freelancer
built the feature, I'm a newbie working on my own app). The parser needs
a conditional which means it will only parse new posts in a feed if they
have a date_published which is later than the time at which the Source
(the applications model naming convention for a feed URL) was created
at, i.e. date_published later than source.created_at. [1] is the
original script, [2] is my failed attempt (but maybe it gives you an
idea of what I've been trying to do);

[1]

if feed
  feed.items.each do |item|
    item_date =
DateTime.parse(item.date_published.to_s).strftime("%Y-%m-%d %H:%M:%S")

    if NewsItem.find(:first, :conditions => {:published_at => item_date,
:source_id => source.id})
      RAILS_DEFAULT_LOGGER.info("RssReader: Already scraped
#{item.url}")
    else
      news_item = NewsItem.new(
        :title => item.title,
        :url => item.url,
        :excerpt => item.content.....

[2]

if feed
  feed.items.each do |item|
    item_date =
DateTime.parse(item.date_published.to_s).strftime("%Y-%m-%d %H:%M:%S")

    # the new conditional
    if item_date < source.created_at
      RAILS_DEFAULT_LOGGER.info("RssReader: No new NewsItems since the
Source was created")
    elsif NewsItem.find(:first, :conditions => {:published_at =>
item_date, :source_id => source.id})
      RAILS_DEFAULT_LOGGER.info("RssReader: Already scraped
#{item.url}")
    else
      news_item = NewsItem.new(
        :title => item.title,
        :url => item.url,
        :excerpt => item.content....

Any ideas? I'm presuming the above doesn't work because it's a
comparison of two strings. However, a friend helped me to try many
different types of DateTime.parse and Time.parse on the
source.created_at in the conditional, to get a fair comparison of
DateTime formats, but none of them worked. We also followed some trial &
error by grabbing the output of different DateTime.parse methods in the
log, but we couldn't find one that works (to_i didn't seem to do it).

Is there a Rails 'later_than' help or a guide to successful DateTime
comparisons out there? Or does anyone know the correct approach for
this?