statistics model for sentiment meter

I want to build a sentiment meter based on youtubes views and likes. Topic is stock market and I intend to build a score for bulishness for a particular stock.
I need a simple formula that will also take into account video publish date so that the influence on a particular video will have a decreasing participation in the scor as the time goes. I know I ask too much, I only need some reference to where I can dig more.

Thanks for any feedback