Is it possible to view stats in the past?

That depends on what you mean, for the custom data uploads you can feed it anything. For Google Analytics you can view the stats (e.g. pageviews) for today (partial), yesterday, past 7 days & past 30 days and compare them to the previous period e.g. page views in the past 30 days are up 12.5% on the previous 30 days.

However, we don't store data (besides temporarily caching it) so if you want to view history it has to be present in the existing feed.

When you query the feed, can you pass in date/time ranges? So, we could query last month?

There are 2 types of feed we accept. The first is a standard connection to one of a variety of web services such as Google Analytics where you authenticate (in this case via OAuth) then choose what you want to display. The second type is custom data uploads where you simply point widgets on your dashboard to an XML/JSON source with an explicitly defined structure and we pull it on a regular basis.

The first type use logic which we have defined up front to extract the appropriate information for display. We apply little or no logic on the custom data feeds as that is assumed to have been carried out by the provider, usually when the XML/JSON file is generated.

If you have an obscure source of data, or it's sourced from a service we don't yet extract from, then I'd suggest writing a script to extract and massage the data into a format we accept and point your dashboard at it. We have definitions and example scripts in our support forum and I'm more than happy to give you a hand setting it up if that would be useful for you.