index.js

const is = (interval, cycle) =>
    cycle >= interval ? Math.round(cycle / interval) : 0;

export default function(time, now) {
    if (!now) {
        now = Date.now();
    }
    const secs = (now - time) / 1000;
    const mins = is(60, secs);
    const hours = is(60, mins);
    const days = is(24, hours);
    const weeks = is(7, days);
    const months = is(30, days);
    const years = is(12, months);

    let amt = years;
    let cycle = 'year';

    if (secs <= 1) {
        return 'just now';
    } else if (years > 0) {
        amt = years;
        cycle = 'year';
    } else if (months > 0) {
        amt = months;
        cycle = 'month';
    } else if (weeks > 0) {
        amt = weeks;
        cycle = 'week';
    } else if (days > 0) {
        amt = days;
        cycle = 'day';
    } else if (hours > 0) {
        amt = hours;
        cycle = 'hour';
    } else if (mins > 0) {
        amt = mins;
        cycle = 'minute';
    } else if (secs > 0) {
        amt = secs;
        cycle = 'second';
    }

    const v = Math.round(amt);

    return `${v === 1 ? (amt === hours ? 'an' : 'a') : v} ${cycle}${v > 1 ? 's' : ''} ago`;
};