The advanced search page has been updated and it shows four more options: find the web pages first indexed in the past day, week, month or in the past 2 months.
If you remove all the uninteresting parameters from the search URL, you'll find that as_qdr is responsible for date restrictions. For example, here's how to restrict a search for [China] to pages first seen by Google's crawler in the past 24 hours:
http://www.google.com/search?q=china&as_qdr=d
Note that you'll only find new web pages and not pages that were updated in the past 24 hours. That means you won't find homepages from popular sites or other frequently-updated pages. If the date range is small, you'll mostly find news and blog posts.
The nice thing is that you can change the value of as_qdr to custom intervals. Here are all the possible values of the as_qdr parameter:
d[number] - past number of days (e.g.: d10)
w[number] - past number of weeks
y[number] - past number of years
For example, http://www.google.com/search?q=china&as_qdr=d10 lets you search for pages that contain "China" and were created in the past 10 days.
A finer control (hours) and an option to sort the results by date would make this feature almost perfect.