Date-relevant methods in Javascript

DateUtil.js

//
// Date related functions
//

function isLeapYear(year) {
	return (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
}

function getLastDay(year, month) {
	var new_year = year; 
	var new_month = month++;
	if (month > 12) 
	{
		new_month -= 12; 
		new_year++;
	}
	var new_date = new Date(new_year, new_month, 1);
	return (new Date(new_date.getTime() - 1000 * 60 * 60 * 24)).getDate();
}

function getMonthDays(year, month) {
	return [31, null, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month] || (isLeapYear(year) ? 29 : 28);
}

function getDaysInMonth(year, month) {
	month = parseInt(month, 10) + 1;
	var temp = new Date(year + "/" + month + "/0");
	return temp.getDate();
}

function getWeekNumber(y, m, d) {
	var now = new Date(y, m - 1, d),
		year = now.getFullYear(),
		month = now.getMonth(),
		days = now.getDate();

	for (var i = 0; i < month; i++) {
		days += getMonthDays(year, i);
	}

	var yearFirstDay = new Date(year, 0, 1).getDay() || 7;
	var week = null;
	if (yearFirstDay == 1) {
		week = Math.ceil(days / yearFirstDay);
	} else {
		days -= (7 - yearFirstDay + 1);
		week = Math.ceil(days / 7) + 1;
	}
	return week;
}

function getDatabaseFormatter(dateinf) {
    if ("object" === typeof dateinf) {
        var y = dateinf.getFullYear();
        var m = dateinf.getMonth() + 1;
        var d = dateinf.getDate();
        return y.toString() + (m < 10 ? ('0' + m) : m).toString() + (d < 10 ? ('0' + d) : d).toString();		
    }
    
    return dateinf;
}

function getDaysBetween (first, second) {
    // Copy date parts of the timestamps, discarding the time parts.
    var one = new Date(first.getYear(), first.getMonth(), first.getDate());
    var two = new Date(second.getYear(), second.getMonth(), second.getDate());

    // Do the math.
    var millisecondsPerDay = 1000 * 60 * 60 * 24;
    var millisBetween = two.getTime() - one.getTime();
    var days = millisBetween / millisecondsPerDay;

    // Round down.
    return Math.floor(days);
}