JSFiddle - React, Tailwind, and code Playground

JavaScript

var now = new Date;
    function today( created_at_day,created_at_month,created_at_year, now ){
     return created_at_day== now.getUTCDate() &&
             created_at_month == (now.getUTCMonth()+1) &&
             created_at_year == now.getUTCFullYear();
    }
    
    // for yesterday, do some calculation here,
    var yest = new Date(now);
    yest = new Date(yest.setDate(yest.getDate() -1));

    function yesterday( created_at_day,created_at_month,created_at_year, yest )
    {
      return created_at_day == yest.getUTCDate() &&
             created_at_month == (yest.getUTCMonth() +1) &&
             created_at_year == yest.getUTCFullYear();
    }
    
   alert("Today: "+today(1,2,2016,now));
   alert("Yesterday: "+yesterday(31,1,2016,yest));