Using RSpec custom matchers to match on the format of a date.

format_based_matcher.rb

RSpec::Matchers.define :be_a_full_datetime do |expected|
  match do |datetime|
    datetime =~ /\w{3} \d+, \d{4} \d{2}:\d{2}/
  end
end

it "has a placed_at date" do
  bid.placed_at.should be_a_full_datetime
end