People will be filling out Yes or No for items 1 through 8, and I want to automatically determine the percentages of Yes values for the columns containing items 1 through 8. It needs to record this specific to the dates in the range so I can see the variance from week to week.

If I am using arrays, I'm unaware of it, but my experience with Excel is rudimentary for now. When you say corce the string to a number, are you referring to the date string?