Right now we're missing aggregated crash data for 2012-07-30, so even though we have the 31st now, the latest available data is from the 29th. When going to https://crash-stats.mozilla.com/topcrasher/byversion/Firefox/17.0a1/1/all the lead-in text says "Top 300 Crashing Signatures. 2012-07-29 through 2012-07-30." and the list below shows the data from the 29th. This is misleading. The text should tell exactly that this is the data from the 29th.
Actually, now that data has been backfilled, it says "2012-07-30 through 2012-07-31". So, it's not wrong but just misleading. This probably means "2012-07-30 00:00 UTC through 2012-07-31 00:00 UTC". We should actually not show the date of a day there from which it doesn't actually contain data, i.e. we should subtract one day (second) from the end date. ;-) (And yes, "2012-07-30 through 2012-07-30" is OK for the 1-day view, we don't need to special-case it there, IMHO.)