I consistently ask Siri (on phone and Watch) what the difference is between two dates, "Hey Siri, what's the difference between August 10 2015 and July 6 1974" and it used to give me an exact answer to the year, month and day. I believe it used to use Wolfram-Alpha. Just recently it stopped working and now does the web search thing. This is annoying, since I know it works and works well and now takes away something Siri used to do. I am not sure if it coincided with the latest OS updates or not. Are you having similar results, thoughts?