Marking up date ranges has yet to be settled at this point.
(You can see the evolving discussion on date ranges on the WhatWG site.)
Your best bet for now is to simply use the ISO 8601 syntax for date ranges as the datetime value in a time tag. In short, just separate the two values with a forward slash.
For example, the 2nd to the 4th of April, 2011 would be:
<time datetime="2011-04-02/2011-04-04">2nd–4th April 2011</time>
Here’s a simpler example representing 2000-2010:
<time datetime="2000/2010">2000–2010</time>
Remember to use an En dash between your dates for proper presentation.