Calculate difference between two timestamps

Here's how you can do it:

const followedAt = new Date('2021-12-25T15:49:57Z');
const currentDate = new Date();
const diffTime = Math.abs(currentDate - followedAt);
const diffDays = Math.ceil(diffTime / (1000 * 60 * 60 * 24)); 
console.log(diffTime + " milliseconds");
console.log(diffDays + " days");

And that's the way to get days and hours:

const followedAt = new Date('2021-12-25T15:49:57Z');
const currentDate = new Date();
const diffTime = Math.abs(currentDate - followedAt);
const diffTotalHours = Math.floor(diffTime / (1000 * 60 * 60)); 
const diffDays = Math.floor(diffTotalHours / 24);
const diffHoursWithoutDays = diffTotalHours % 24;

console.log(`${diffDays} days and ${diffHoursWithoutDays} hours`);