Until now, you could only find images from the past 24 hours, the past week or the past year. If you edited Google's URL, the feature could be used to find pages from the past 10 days, 2 months or other custom periods of time, but you couldn't use it to find pages from 2006.
"Google tries to estimate the publication date for a page by using information such as the date Google first crawled the page," explains Google's help center. Google also uses the dates included in web pages if they are good candidates for the published date. Since Google's date is only an estimation, it should be taken with a grain of salt.
Power tip: The new feature is the only way you can find videos from a certain date range in Google Video. Just restrict the results to "videos".