Amazon already accounts for the date of the review to some extent. If you hover over the overall rating for a product you can read: "Amazon calculates a produc

Amazon already accounts for the date of the review to some extent. If you hover over the overall rating for a product you can read: "Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases."