HTML5 bring in a bunch of new meaningful tags. Amongst those is the time tag. Its purpose, thoughtfully enough, is the give meaning to a date or time.
<time>5:00</time>
But what if you have a date, or even a word that means a point in time. What is the meaning behind that?
<time>5am yesterday</time>
<time>July 4, 2010</time>
To give more meaning to it, you must use the datetime attribute.
<time datetime="2010-05-28T05:00">5am last Tuesday</time>
<time datetime="2010-07-04">July 4, 2010</time>
So we can see that with HTML5, we can give purpose and meaning to time and dates.