What is the date of the day that is 11 weeks and 3 days prior to Oct 31st?

Run your cursor to the lower right corner of the screen where the clock appears and click there. You will get a calendar. Click the left arrow to go back one month. Now you can simply count weeks and days.

TIP: You can also click "Set Date And Time" to see the time anywhere in the world.

Another possibility is to go to google.com and type "calendar calculator".