time_tag

time_tag Date.today  # =>
  <time datetime="2010-11-04">November 04, 2010</time>
time_tag Time.now  # =>
  <time datetime="2010-11-04T17:55:45+01:00">November 04, 2010 17:55</time>
time_tag Date.yesterday, 'Yesterday'  # =>
  <time datetime="2010-11-03">Yesterday</time>
time_tag Date.today, pubdate: true  # =>
  <time datetime="2010-11-04" pubdate="pubdate">November 04, 2010</time>
time_tag Date.today, datetime: Date.today.strftime('%G-W%V') # =>
  <time datetime="2010-W44">November 04, 2010</time>

<%= time_tag Time.now do %>
  <span>Right now</span>
<% end %>
# => <time datetime="2010-11-04T17:55:45+01:00"><span>Right now</span></time>