Human formatted time between dates (JavaScript)

gistfile1.js

/**
 * Given two dates (or one date and assume "now" for the second), convert this to
 * a human-readable string, like "2 months".
 *
 * I use this to put "3 months ago" strings into plugins. My use case has the date
 * coming in as a seconds-only UNIX epoch, so the params are expected at this.
 *
 * @param time1 integer Number of seconds since UNIX epoch
 * @param time2 integer Number of seconds since UNIX epoch
 * @return string
 */
function(time1, time2) {
    // Check/sanitise vars
    time1 = Math.max(0, parseInt(time1));

    if(typeof time2 == "undefined") {
        var now = new Date();
        time2 = Math.floor(now.getTime() / 1000);
    }
    var period = Math.abs(time1 - time2);

    var timespan = 1;
    var format = 'seconds';
    if (period > 31556926) {
        // More than one year
        format = 'years';
        timespan = Math.floor(period / 31556926);
    }
    else if (period > 2629744) {
        // More than one month
        format = 'months';
        timespan = Math.floor(period / 2629744);
    }
    else if (period > 604800) {
        // More than one week
        format = 'weeks';
        timespan = Math.floor(period / 604800);
    }
    else if (period > 86400) {
        // More than one day
        format = 'days';
        timespan = Math.floor(period / 86400);
    }
    else if (period > 3600) {
        // More than one hour
        format = 'hours';
        timespan = Math.floor(period / 3600);
    }
    else if (period > 60) {
        // More than one minute
        format = 'minutes';
        timespan = Math.floor(period / 60);
    }

    // Remove the s
    if(timespan == 1) {
        format = substr(format, 0, -1);
    }

    return timespan + ' ' + format;
};