let time = Date.parse('2018-00-01T00:00:00'); 

let date = new Date();
let now = date.getTime(); 

let result = now - time; 
result = result / (); 
alert(result);

let date = Date.parse(2018); 
let now = new Date();
let result = (now - date)/604800000;
alert(result); 
alert(Math.floor(result)); // полных недель