Untitled

let counter = 0,
    startDate = new Date('December 17, 2015');
    
setTimeout( () => {
    let todayInSeconds = Math.round(Date.now() / 1000);
    let startInSeconds = Math.round(startDate) / 1000);
    counter = todayInSeconds - startInSeconds;
}, 1000);

console.log(counter);