var it_departureDate = new Date();
it_departureDate = new Date(it_departureDate.getTime() + (3 * 24 * 60 * 60 * 1000));
var it_returnDate = new Date();
it_returnDate = new Date(it_returnDate.getTime() + (10 * 24 * 60 * 60 * 1000));
var monthName = ["", "Janv.", "Fév.", "Mars", "Avril", "Mai", "Juin", "Juil.", "Août", "Sept.", "Oct.", "Nov.", "Déc."];
	
function it_getDaysInMonth( m, y ) {
	var it_monthdays = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
	if (m != 2) {
		return it_monthdays[m];
	} else {
		return ((y % 4 == 0 && y % 100 != 0) || y % 400 == 0 ? 29 : 28);
	}
}

function dateComboBuilder(date, fieldName){
	var day = date.getDate();
	var month = date.getMonth() + 1;
	var year = date.getFullYear();
	if (fieldName == 'da') {
		for (var i=1; i <= it_getDaysInMonth(month, date.getFullYear()); i++){
			var str = i < 10 ? "0" + i : "" + i;
			document.write('<option value="' + str + '" ');
			if (i == day) {
				document.write("selected");
			}
			document.write('>' + str + '</option>');
		}
	} else if (fieldName == 'mo') {
		for (var i=1; i <= 12 ;i++){
			var str = i < 10 ? "0" + i : "" + i;
			document.write('<option value="' + str + '" ');
			if (i == month) {
				document.write("selected");
			}
			document.write('>' + monthName[i] + '</option>');
		}
	} else if (fieldName == 'yr') {
		for (var i=year; i <= (year+1) ;i++){
			document.write('<option value="'+i+'">'+i+'</option>');
		}
	}
}
function buildOptions(from, to, defaultNumber){
	for (var i=from; i <= to; i++){
		document.write('<option value="' + i + '" ');
		if (i == defaultNumber) {
			document.write("selected");
		}
		document.write('>' + i + '</option>');
	}
}

<!-- hotel part -->
function checkCity() {
	var citySelect = document.IT_hot_Form.citySelect;
	var cityInput  = document.IT_hot_Form.cityName;

	if (cityInput.value != "") {
		return true;
	} else {
		alert("Veuillez préciser la ville de destination");
		return false;
	}
}

function updateCityInput() {
	var citySelect    = document.IT_hot_Form.citySelect;
	var cityInput     = document.IT_hot_Form.cityName;
	var countryInput  = document.IT_hot_Form.countryName;
	var tbCountryCity = document.IT_hot_Form.citySelect.value.split('§');

	if (citySelect.value != "empty" && citySelect.value != "C") {
		cityInput.value    = tbCountryCity[1];
		countryInput.value = tbCountryCity[0];
	}
}
function clearCitySelect() {
	var citySelect    = document.IT_hot_Form.citySelect;
	var cityInput     = document.IT_hot_Form.cityName;
	
	if (cityInput.value != "" && citySelect.value != "empty") {
		citySelect.value = "empty";
	}
}
<!-- car part -->
function hourBuilder(defaultHour){
	for (var i=0; i <= 24; i++){
		var hour = '';
		(i < 10)? hour = '0'+i: hour = i;
		document.write('<option value="' + hour + '" ');
		if (hour == defaultHour) {
			document.write("selected");
		}
		document.write('>' + hour + '</option>');
	}
}