Date Of Birth (DOB) Validation - PHP & JS pulled from a project - these snippets have not been tested on their own.

index.php

	<script type="text/javascript" src="/js/jquery-1.3.2.min.js"></script>
	<script type="text/javascript" src="/js/jquery.validate.min.js"></script>
	<script type="text/javascript" src="/js/jquery.validate.additional.js"></script>
	
	<style>
		li.error, label.error, p.error{color:#F00;}
		#entryForm ol{list-style-type:none;}
		#entryForm label{display:block; margin-top:6px;}
		#entryForm label.error{margin-top:0px;}
	</style>
	
	
	<form action="submit.php" method="POST" id="entryForm">
		<div id="entryFormError" <?php if(!isset($_GET['errors'])){ print 'style="display:none;"'; }?>>
			<p class="error">There was an error with your entry:</p>
			<?php
				if(isset($_GET['errors']) && is_array($_GET['errors']) && !empty($_GET['errors']))
				{
					print '<ol>';
					foreach($_GET['errors'] as $tErrorMessage)
					{
						print '<li class="error">' . $tErrorMessage . '</li>';
					}
					print '</ol>';
				}
			?>
		</div>
		<ol>
			<li>
				<label for="entryDay">
					DATE OF BIRTH:
				</label>
				<div class="select">
					<select id="entryDay" name="entry[day]">
						<?php 
							$tDay = (isset($_GET['day'])) ? $_GET['day'] : '';
							$tSelected = (empty($tDay)) ? 'selected="selected"' : '';
							printf('<option %s value="" label="Day">Day</option>', $tSelected);
							for($i=1; $i <= 31; $i++)
							{
								$tFormated = str_pad($i, 2, 0, STR_PAD_LEFT);
								$tSelected = ($tDay == $tFormated ) ? 'selected="selected"' : ''; 
								printf('<option %s value="%2$s" label="%2$s">%2$s</option>', $tSelected, $tFormated);
							}
						?>
					</select>
					<select id="entryMonth" name="entry[month]">
						<?php 
							$tMonth = (isset($_GET['month'])) ? $_GET['month'] : '';
							$tSelected = (empty($tMonth)) ? 'selected="selected"' : '';
							printf('<option %s value="" label="Month">Month</option>', $tSelected);
							$tMonths = array(
								'Jan' => '01',
								'Feb' => '02',
								'Mar' => '03',
								'Apr' => '04',
								'May' => '05',
								'Jun' => '06',
								'Jul' => '07',
								'Aug' => '08',
								'Sep' => '09',
								'Oct' => '10',
								'Nov' => '11',
								'Dec' => '12'
							);
							foreach($tMonths as $tKey => $tValue)
							{
								$tSelected = ($tMonth == $tValue ) ? 'selected="selected"' : ''; 
								printf('<option %s value="%2$s" label="%3$s">%3$s</option>', $tSelected, $tValue, $tKey);
							}
						?>
					</select>
					<select id="entryYear" name="entry[year]">
						<?php 
							$tYear = (isset($_GET['year'])) ? $_GET['year'] : '';
							$tSelected = (empty($tYear)) ? 'selected="selected"' : '';
							printf('<option %s value="" label="Year">Year</option>', $tSelected);
							// 1993 will turn 17 in 2010
							for($i=1900; $i <= 1993; $i++)
							{
								$tSelected = ($tYear == $i ) ? 'selected="selected"' : ''; 
								printf('<option %s value="%2$s" label="%2$s">%2$s</option>', $tSelected, $i);
							}
						?>
					</select>
				</div>
			</li>
			<li>
				<button type="submit" id="btnSubmit">ENTER</button>
			</li>
		</ol>
	</form>
	
	<script type="text/javascript">
		jQuery(function($){
			var validator = $("#entryForm").validate({
				rules: {
					"entry[year]":				{ required:true, checkdate:'#entryYear #entryMonth #entryDay' }
				},
					"entry[year]":				{ required:'Your date of birth is required', checkdate:'Please enter a valid date of birth' }
				}
			});
		});
	</script>

jquery.validate.additional.js

// Checkdate - Validate a Gregorian date, from an ISO formatted date string (YYYY-MM-DD) - as per PHP function checkdate()
// @param can pass in either:
		// bool TRUE to validate a single field
		// OR
		// a string of 3 DOM ID's in the sequence Year, Month, Day e.g. #form_user_dob_year #form_user_dob_month #form_user_dob_day
		// Note: only assign the validation rule to 1 of the 3 fields. e.g "user[dob][Year]":	{ required:true, checkdate:'#form_user_dob_year #form_user_dob_month #form_user_dob_day' },
jQuery.validator.addMethod("checkdate", function(value, element, param) {
	
	// if validating 3 fields, replace the validating 'value' with the 3 field values, in ISO format YYYY-MM-DD
	if(param && param !== true)
	{
		var paramArray = param.split(' ');
		var pYear = $(paramArray[0]).val();
		var pMonth = $(paramArray[1]).val();
		var pDay = $(paramArray[2]).val();
		value = pYear + '-' + pMonth + '-' + pDay;
	}
	
	// validate against jquery validator's dateISO: check
	// http://docs.jquery.com/Plugins/Validation/Methods/dateISO
	var validDate =	/^\d{4}[\/-]\d{1,2}[\/-]\d{1,2}$/.test(value);
	
	if(validDate)
	{
		var dateArray = value.split('-');
		var year = dateArray[0];
		var month = dateArray[1];
		var day = dateArray[2];

		// PHP.js checkdate validation
		// http://kevin.vanzonneveld.net
		// +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
		// +   improved by: Pyerre
		// *     example 1: checkdate(12, 31, 2000);
		// *     returns 1: true
		// *     example 2: checkdate(2, 29, 2001);
		// *     returns 2: false
		// *     example 3: checkdate(03, 31, 2008);
		// *     returns 3: true
		// *     example 4: checkdate(1, 390, 2000);
		// *     returns 4: false
		var myDate = new Date();
		myDate.setFullYear( year, (month - 1), day );
		var success = month >= 1 && month <= 12 && year >= 1 && year <= 32767 && ((myDate.getMonth()+1) == month && day<32);

		return this.optional(element) || success;
	}
}, jQuery.validator.messages.checkdate);

submit.php

<?php
	//    phpinfo();
	
	/**** Init ****/
	$tFilePath = rtrim(dirname($_SERVER["PHP_SELF"]), '/\\');
	$tSiteUrl = $_SERVER["SERVER_NAME"];
	$tCurrentUrl = 'http://' . $tSiteUrl . $tFilePath;
	$tData = ( isset($_POST['entry']) ) ? $_POST['entry'] : FALSE;
	
	/**** Validate Required Data ****/
	// Return No Data Error
	if(empty($tData) || !is_array($tData))
	{
		$tErrors['errors'][] = 'No data submitted';
		triggerError( $tErrors );
	}
	
	// Validate Missing Required Fields
	$tRequiredFields = array( 
		'day' => 'Your DOB day is required', 
		'month' => 'Your DOB month is required', 
		'year' => 'Your DOB year is required'
	);
	$tErrors = false;
	foreach($tRequiredFields as $tKey => $tValue)
	{
		if( !isset($tData[$tKey]) || false === $tData[$tKey] || '' === $tData[$tKey] )
		{
			$tErrors['errors'][] = $tRequiredFields[$tKey];
		}
	}
	
	// Return Missing Field Errors
	if($tErrors)
	{
		triggerError( $tErrors );
	}
	
	/*** Validate Special Conditions for Data ***/
	// Invalid DOB
	$VALID_DATEISO  = '/^\d{4}(?:-\d{1,2}){2}$/';
	$tData['dob'] = $tData['year'] . '-' . $tData['month'] . '-' . $tData['day'];

	// validate date format YYYY-MM-DD
	if ( !preg_match($VALID_DATEISO, $tData['dob']) )
	{
		$tErrors['errors'][] = 'Please enter a valid date of birth';
	} 
	else 
	{
		// validate date is real (i.e. there are not 31 days in February)
		$tDateArray = explode('-', $tData['dob']);
		$tYear 	= (int)$tDateArray[0];
		$tMonth = (int)$tDateArray[1];
		$tDay 	= (int)$tDateArray[2];
		// Validate a Gregorian date
		if ( !checkdate($tMonth, $tDay, $tYear) )
		{
			$tErrors['errors'][] = 'Please enter a valid date for date of birth';
		}
	}

	// Return Validation Errors
	if($tErrors)
	{
		triggerError( $tErrors );
	}
	

	/**
	 * FUNCTIONS
	**/
	function redirectURL( $tURL )
	{
		header("location: $tURL");
		exit;
	}
	
	function triggerError( $tErrors )
	{
		global $tCurrentUrl;
		global $tData;
		$tParams = http_build_query($tErrors);
		$tParams .= '&' . http_build_query($tData);
		$tURL = $tCurrentUrl . '/?' . $tParams;
		redirectURL($tURL);
	}
?>

That looks familiar ;)

^_^