JS replacement functions for timestamp <==> date parts without using the Date object

DatePartFunctions.js

var d365 = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365 ];
var d366 = [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366 ];

function parts2ts(year, month, day, hour, minute, second, millisecond) {
    if (month < 0 || month > 11) {
        return NaN;
    }

    var leap = year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
    var days = leap ? d366 : d365;
    if (day < 1 || day > days[month + 1] - days[month]) {
        return NaN;
    }

    var f = Math.floor;
    var y = year - 1;
    var daysSinceEpoch = y * 365 + f(y / 4) - f(y / 100) + f(y / 400) + days[month] + day - 719163;

    return (daysSinceEpoch * 8640 * 10000) + (hour * 3600 * 1000) + (minute * 60000) + (second * 1000) + millisecond;
}

function ts2parts(ts) {
    var f = Math.floor;
    var n = f(ts / 864e5) + 719162;
    var y400 = f(n / 146097);
    n -= y400 * 146097;

    var y100 = f(n / 36524);
    if (y100 == 4) y100 = 3;
    n -= y100 * 36524;

    var y4 = f(n / 1461);
    n -= y4 * 1461;

    var y1 = f(n / 365);
    if (y1 == 4) y1 = 3;
    n -= y1 * 365;

    var year = y400 * 400 + y100 * 100 + y4 * 4 + y1 + 1;

    var leap = y1 == 3 && (y4 != 24 || y100 == 3);
    var days = leap ? d366 : d365;
    var month = n >> 5;
    while (n >= days[month + 1])
        month++;

    var day = n - days[month] + 1;

    var hour = (Math.floor((ts % 864e5) / 36e5) + 24) % 24;
    var minute = (Math.floor((ts % 36e5) / 6e4) + 60) % 60;
    var second = (Math.floor((ts % 6e4) / 1e3) + 60) % 60;
    var millisecond = ((ts % 1e3) + 1e3) % 1e3;

    return [year, month, day, hour, minute, second, millisecond];
}

function parts2ts_native(year, month, day, hour, minute, second, millisecond) {

    return Date.UTC(year, month, day, hour, minute, second, millisecond);
}


function ts2parts_native(ts) {

    var dt = new Date(ts);

    return [dt.getUTCFullYear(), dt.getUTCMonth(), dt.getUTCDate(), dt.getUTCHours(), dt.getUTCMinutes(), dt.getUTCSeconds(), dt.getUTCMilliseconds()];
}

I'm also thinking about this with regard to moment.js

The "why is this faster" may be in part that I'm not handling underflow/overflow scenarios, and that I'm computing the values all at once in a single pass, rather than one part at a time.

Updated to fix floating point math errors, and errors with flooring negative values.

Updated again, apparently I botched the time parts of ts2parts. oops!