my bad, I checked Wikipedia and got deceived by the phrase “[…] officially launched at the Galaxy Nexus and Ice Cream Sandwich release event on 19 October 2011” but then it says only the SDK was released that day. so, one month difference instead of one week.
http://www.quora.com/What-proportion-of-all-iPhone-owners-us...
http://www.marco.org/2011/11/30/more-ios-device-and-os-versi...
for reference, iOS 5 was released October 12, Ice Cream Sandwich on October 19.