text: 
  const agendaBtn = document.querySelector(".agendaBtn") ;
  const showDate  = new Date("Fri May 18 2018 00:00:00 GMT+0200 (CEST)") ;
  const hideDate  = new Date("Sat May 19 2018 23:59:59 GMT+0200 (CEST)") ;
  
  window.setInterval( function()  {
  
      let current = new Date();
  
      if( current.getTime() > showDate.getTime() && current.getTime() < hideDate.getTime() )    {
          agendaBtn.style.display="block" ;
      }
      else    {
          agendaBtn.style.display="none" ;
      }
    
  }, 1000) ;
  