If you're interested in dates/calendars, check out the MCAL stuff.

There are two world calculations for the date of Easter.  The Easter date
function should account for this; one used (generally) by the Western
world and one (generally) used by the Eastern (the official date used by
the East Orthodox Church).

if, like me, you don't have a PHP build that includes the cal functions,
you may want to use this function for sorting out leap year.

function days_in_feb($year){

	//$year must be YYYY
	//[gregorian] leap year math :
	
	if ($year < 0) $year++;
	$year += 4800;

    if ( ($year % 4) == 0) {
		if (($year % 100) == 0) {
		    if (($year % 400) == 0) {
				return(29);
		    } else {
				return(28);
		    }
		} else {
		    return(29);
		}
    } else {
		return(28);
    }
}

of course the next leap year isn't until the end of the century but this
makes for timeless code I guess ...or if you are using 2000 in your dates
or are going far back in time, etc, it is necessary.

A correction to the manual:
In PHP 4.0.5 and above Calendar, support is built-in. You don't need to
load any additional extensions in order to use these functions. 

Thanks,

Vikas

Find the number of days in a month:

// Month
$m = 2;

// Year
$y = 2000;

// Seed it with 31
$daysinmonth = 31;

while (checkdate($m, $daysinmonth, $y) == false) {
    $daysinmonth -= 1;
}

// Now $daysinmonth equals the correct number of days in the month.

function days_in_month ( $monat, $jahr ) {
 return 32 - date ("d", mktime (0,0,0,$monat,32,$jahr));
}