Most of the time you look at a dashboard because you want to see the current state of information. Sometimes though it can be useful to look at a point in time in history for that dashboard, almost like a report.
You can do this by using an addition to the url. Any dashboard will look something like '<subdomain>.exosite.com/views/00000000/0000000'. That second set of numbers is the dashboard ID. If you add '?time=<timestamp>' to it you can look back in time. <timestamp> must be a unix timestamp. (Quickly find a timestamp using this tool http://www.unixtimestamp.com/index.php)
So, here is an example:
Here is my plant monitor (realtime):
Here is my plant monitor on January 31, 2012: