Calculating the difference between dates in years, months, and days

<?php
/*
	License:  do whatever you want with this code
	
	Disclaimer:  This code works well on my system, but may not on yours.  Use
	with circumspection and trepidation.  If this code blows up your system, 
	I recommend vituperation.
*/



/*
	function smoothdate simply takes a year, month, and a day, and
	concatenates them in the form YYYYMMDD
	
	the function date_difference uses this function
*/

function smoothdate ($year, $month, $day)
{
	return sprintf ('%04d', $year) . sprintf ('%02d', $month) . sprintf ('%02d', $day);
}


/*
	function date_difference calculates the difference between two dates in
	years, months, and days.  There is a ColdFusion funtion called, I
	believe, date_diff() which performs a similar function.
	
	It does not make use of 32-bit unix timestamps, so it will work for dates
	outside the range 1970-01-01 through 2038-01-19.  This function works by
	taking the earlier date finding the maximum number of times it can
	increment the years, months, and days (in that order) before reaching
	the second date.  The function does take yeap years into account, but does
	not take into account the 10 days removed from the calendar (specifically
	October 5 through October 14, 1582) by Pope Gregory to fix calendar drift.
	
	As input, it requires two associative arrays of the form:
	array (	'year' => year_value,
			'month' => month_value.
			'day' => day_value)
	
	The first input array is the earlier date, the second the later date.  It
	will check to see that the two dates are well-formed, and that the first
	date is earlier than the second.
	
	If the function can successfully calculate the difference, it will return
	an array of the form:
	array (	'years' => number_of_years_different,
			'months' => number_of_months_different,
			'days' => number_of_days_different)
			
	If the function cannot calculate the difference, it will return FALSE.
	
*/

function date_difference ($first, $second)
{
	$month_lengths = array (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

	$retval = FALSE;

	if (	checkdate($first['month'], $first['day'], $first['year']) &&
			checkdate($second['month'], $second['day'], $second['year'])
		)
	{
		$start = smoothdate ($first['year'], $first['month'], $first['day']);
		$target = smoothdate ($second['year'], $second['month'], $second['day']);
							
		if ($start <= $target)
		{
			$add_year = 0;
			while (smoothdate ($first['year']+ 1, $first['month'], $first['day']) <= $target)
			{
				$add_year++;
				$first['year']++;
			}
																											
			$add_month = 0;
			while (smoothdate ($first['year'], $first['month'] + 1, $first['day']) <= $target)
			{
				$add_month++;
				$first['month']++;
				
				if ($first['month'] > 12)
				{
					$first['year']++;
					$first['month'] = 1;
				}
			}
																																											
			$add_day = 0;
			while (smoothdate ($first['year'], $first['month'], $first['day'] + 1) <= $target)
			{
				if (($first['year'] % 100 == 0) && ($first['year'] % 400 == 0))
				{
					$month_lengths[1] = 29;
				}
				else
				{
					if ($first['year'] % 4 == 0)
					{
						$month_lengths[1] = 29;
					}
				}
				
				$add_day++;
				$first['day']++;
				if ($first['day'] > $month_lengths[$first['month'] - 1])
				{
					$first['month']++;
					$first['day'] = 1;
					
					if ($first['month'] > 12)
					{
						$first['month'] = 1;
					}
				}
				
			}
																																																														
			$retval = array ('years' => $add_year, 'months' => $add_month, 'days' => $add_day);
		}
	}
																																																																				
	return $retval;
}


/*
	This code is merely an example of use of the function
*/

print '<pre>';
$begin = array ('year' => 2001, 'month' => 3, 'day' => 14);
$end = array ('year' => 2004, 'month' => 3, 'day' => 14);

$foo = date_difference ($begin, $end);
if ($foo !== FALSE)
{
	print_r ($foo);
}
else
{
	print 'FALSE';
}

?>