Javascript library for human-friendly relative date formatting

I wrote moment.js, a date library that does this. It's about 5KB (2011) 52KB (2019), and works in browsers and in Node. It's also probably the most popular and famous date library for JavaScript.

It supports timeago, formatting, parsing, querying, manipulating, i18n, etc.

Timeago (relative time) for dates in the past is done with moment().fromNow(). For example, to display January 1, 2019 in the timeago format:

let date = moment("2019-01-01", "YYYY-MM-DD");
console.log(date.fromNow());

<script src="https://momentjs.com/downloads/moment.min.js"></script>

The timeago strings are customizable with moment.updateLocale(), so you can change them how you see fit.

The cutoffs are not what the question requests ("5 weeks" vs. "1 month"), but it is documented as to which strings are used for what time range.

Since I wrote this answer, a well known library available is moment.js.

There are libraries available, but it is trivial to implement it yourself. Just use a handful of conditions.

Assume date is an instantiated Date object for the time you want to make a comparison against.

// Make a fuzzy time
var delta = Math.round((+new Date - date) / 1000);

var minute = 60,
    hour = minute * 60,
    day = hour * 24,
    week = day * 7;

var fuzzy;

if (delta < 30) {
    fuzzy = 'just then.';
} else if (delta < minute) {
    fuzzy = delta + ' seconds ago.';
} else if (delta < 2 * minute) {
    fuzzy = 'a minute ago.'
} else if (delta < hour) {
    fuzzy = Math.floor(delta / minute) + ' minutes ago.';
} else if (Math.floor(delta / hour) == 1) {
    fuzzy = '1 hour ago.'
} else if (delta < day) {
    fuzzy = Math.floor(delta / hour) + ' hours ago.';
} else if (delta < day * 2) {
    fuzzy = 'yesterday';
}

You would need to adapt this to handle future dates.

Here's something from the John Resig - http://ejohn.org/blog/javascript-pretty-date/

EDIT (6/27/2014): Following up on the comment from Sumurai8 - though the linked page still works, here is the excerpt for the pretty.js linked to from the article above:

pretty.js

/*
 * JavaScript Pretty Date
 * Copyright (c) 2011 John Resig (ejohn.org)
 * Licensed under the MIT and GPL licenses.
 */

// Takes an ISO time and returns a string representing how
// long ago the date represents.
function prettyDate(time) {
    var date = new Date((time || "").replace(/-/g, "/").replace(/[TZ]/g, " ")),
        diff = (((new Date()).getTime() - date.getTime()) / 1000),
        day_diff = Math.floor(diff / 86400);

    if (isNaN(day_diff) || day_diff < 0 || day_diff >= 31) return;

    return day_diff == 0 && (
    diff < 60 && "just now" || diff < 120 && "1 minute ago" || diff < 3600 && Math.floor(diff / 60) + " minutes ago" || diff < 7200 && "1 hour ago" || diff < 86400 && Math.floor(diff / 3600) + " hours ago") || day_diff == 1 && "Yesterday" || day_diff < 7 && day_diff + " days ago" || day_diff < 31 && Math.ceil(day_diff / 7) + " weeks ago";
}

// If jQuery is included in the page, adds a jQuery plugin to handle it as well
if (typeof jQuery != "undefined") jQuery.fn.prettyDate = function() {
    return this.each(function() {
        var date = prettyDate(this.title);
        if (date) jQuery(this).text(date);
    });
};

Usage:

prettyDate("2008-01-28T20:24:17Z") // => "2 hours ago"
prettyDate("2008-01-27T22:24:17Z") // => "Yesterday"
prettyDate("2008-01-26T22:24:17Z") // => "2 days ago"
prettyDate("2008-01-14T22:24:17Z") // => "2 weeks ago"
prettyDate("2007-12-15T22:24:17Z") // => undefined

Excerpt from the article on usage:

Example Usage

In the following examples I make all the anchors on the site, that have a title with a date in it, have a pretty date as their inner text. Additionally, I continue to update the links every 5 seconds after the page has loaded.

With JavaScript:

function prettyLinks(){
    var links = document.getElementsByTagName("a");
    for ( var i = 0; i < links.length; i++ )
        if ( links[i].title ) {
            var date = prettyDate(links[i].title);
            if ( date )
                links[i].innerHTML = date;
        }
}
prettyLinks();
setInterval(prettyLinks, 5000);

With jQuery:

$("a").prettyDate();
setInterval(function(){ $("a").prettyDate(); }, 5000);

Faiz: Made some changes to the original code, bug fixes and improvements.

function prettyDate(time) {
    var date = new Date((time || "").replace(/-/g, "/").replace(/[TZ]/g, " ")),
        diff = (((new Date()).getTime() - date.getTime()) / 1000),
        day_diff = Math.floor(diff / 86400);
    var year = date.getFullYear(),
        month = date.getMonth()+1,
        day = date.getDate();

    if (isNaN(day_diff) || day_diff < 0 || day_diff >= 31)
        return (
            year.toString()+'-'
            +((month<10) ? '0'+month.toString() : month.toString())+'-'
            +((day<10) ? '0'+day.toString() : day.toString())
        );

    var r =
    ( 
        (
            day_diff == 0 && 
            (
                (diff < 60 && "just now")
                || (diff < 120 && "1 minute ago")
                || (diff < 3600 && Math.floor(diff / 60) + " minutes ago")
                || (diff < 7200 && "1 hour ago")
                || (diff < 86400 && Math.floor(diff / 3600) + " hours ago")
            )
        )
        || (day_diff == 1 && "Yesterday")
        || (day_diff < 7 && day_diff + " days ago")
        || (day_diff < 31 && Math.ceil(day_diff / 7) + " weeks ago")
    );
    return r;
}