> gearthblog.com/blog/archives/2011/03/how_google_earth_displays_dates_on.html
For standard satellite images, it’s simply the date the imagery was taken, which makes sense. Easy enough. The discrepancies arise when Google loads imagery for a large area from a commercial aerial provider. In those cases, they’re given a range of dates for the imagery. The date you see on the screen is the “oldest known date” for that imagery, while the tic mark in the Historical Imagery sliders is the “newest known date”. In many cases, those date ranges can be up to a few months apart.
To confuse it further, some providers don’t even have exact dates for a batch of imagery; they might simply say April-June, 2010. In those cases, Google considers that to be April 1 – June 30, 2010, and then displays the date as explained in the previous paragraph