timeSince function in javascript

gistfile1.js

function timeSince(timeStamp, options) {
    if (typeof timeStamp === "object") {
        options = timeStamp;
        timeStamp = undefined;
    }
    options = options || {};
    _.defaults(options, {
        compareDate: +new Date(),
        timeChunk: undefined,
        maxUnit: "year",
        unixUptime: false,
        max_levels: 3,
        timeStamp: timeStamp || 0
    });
    var remaining = (options.timeChunk !== undefined) ? options.timeChunk : options.compareDate - options.timeStamp;
    var string = "";
    var separator = ", ";
    var level = 0;
    var max_levels = options.max_levels;
    var milli_per_second = 1000;
    var milli_per_minute = milli_per_second * 60;
    var milli_per_hour = milli_per_minute * 60;
    var milli_per_day = milli_per_hour * 24;
    var milli_per_week = milli_per_day * 7;
    var milli_per_month = milli_per_week * 4;
    var milli_per_year = milli_per_day * 365;
    
    if (options.unixUptime) {
        var days = Math.floor(remaining / milli_per_day);
        remaining -= days*milli_per_day;
        var hours = Math.floor(remaining / milli_per_hour);
        remaining -= hours*milli_per_hour;
        var minutes = Math.round(remaining / milli_per_minute);
        
        if (days === 0) {
            minutes = Math.floor(remaining / milli_per_minute);
            remaining -= minutes*milli_per_minute;
            var seconds = Math.round(remaining / 1000);
            string = (hours < 10 ? "0" : "")+hours+':'+(minutes < 10 ? "0" : "")+minutes+':'+(seconds < 10 ? "0" : "")+seconds;
        }
        else {
            string = days + " days, " + hours.toString() + ":" + (minutes < 10 ? "0" : "") + minutes.toString();
        }
        
    } else {
        var levels = [
            { plural: "years", singular: "year", ms: milli_per_year },
            { plural: "months", singular: "month", ms: milli_per_month },
            { plural: "weeks", singular: "week", ms: milli_per_week },
            { plural: "days", singular: "day", ms: milli_per_day },
            { plural: "hours", singular: "hour", ms: milli_per_hour },
            { plural: "minutes", singular: "minute", ms: milli_per_minute },
            { plural: "seconds", singular: "second", ms: milli_per_second }
        ];

        var crossedThreshold = false;
        for (var i=0; i < levels.length; i++) {
            if ( options.maxUnit === levels[i].singular ) crossedThreshold = true;
            if ( remaining < levels[i].ms || !crossedThreshold ) continue;
            level++;
            var num = Math.floor( remaining / levels[i].ms );
            var label = num == 1 ? levels[i].singular : levels[i].plural ;
            string += num + " " + label + separator;
            remaining %= levels[i].ms;
            if ( level >= max_levels ) break;
        };
        string = string.substring(0, string.length - separator.length);
    }
    
    
    return string;
}