A Google Groups thread has a webmaster questioning the date shown in Google Webmaster Tools that shows the "Home page crawl" summary. In short, the data may be older and slower to update than other tools you have in your arsenal. Google's JohnMu, confirmed that.
Here is a screen capture of Google Webmaster Tools for this site:
Here is a screen capture of the Google cache for this site:
Both were taken almost exactly the same time, but as you can see, the Google cache has a more recent date. Of course, for most SEOs, this is nothing new. But for those that do not know about this, it is good to know that you have nothing to worry about.
Even the cache date above is a bit slow, because I know Google crawled and indexed articles from this site after that time listed. The date may mean a few things, but it does not 100% mean the last date Google crawled and indexed you content.
Alternatives? I'll quote John:
Sometimes it can take a bit of time for Webmaster Tools to update this information. Checking the cached version of the URL (or checking your server logs) may be more accurate if you need to find an exact date.
Forum discussion at Google Groups.