timeSince.js

function timeSince(timeStamp) {
    var now = new Date(),
        secondsPast = (now.getTime() - timeStamp.getTime()) / 1000;
    if(secondsPast < 60){
        return [-parseInt(secondsPast), 'seconds']
    }
    if(secondsPast < 3600){
        return [-parseInt(secondsPast/60), 'minutes']
    }
    if(secondsPast <= 86400){
        return [-parseInt(secondsPast/3600), 'hours']
    }
    if(secondsPast <= 86400 * 7) {
        return [-parseInt(secondsPast/86400), 'day']
    }
    return [-parseInt(secondsPast/(86400 * 7)), 'week']
}

function formatRelativeTime(date, locale) {
    return new Intl.RelativeTimeFormat(locale).format(...timeSince(date))
}