list check expiry date for calendar

i've been trying to find out how to calculate for expiry date in php.
basically i've 2 date signup_date and expiry_date. user have to key in singup_date but for expiry_date i wanted to come out calculating by 3month, 6month, 1year and 2year etc. i dunno how to calculate for those using php.
i found out this one

but that was not working.

 

something like this

$years = 1;
$months = 6;
$days = ($years * 365) + ($months * 30);
// or
$days = ($years * 365) + (365/$months);

$expiry_date= time() + (
    $days * 
    24 /* hours */ *
    60 /* minutes */ *
    60 /* seconds */

);

PHP:

 

Rather use date and mktime (see below) with php lesser than 5.3

With php 5.3 and up you can use "Datetime" and "Dateinterval"

// Example for php < 5.3
echo "Today: " 
.date('Y-m-d') ."\n";

$nMonths = 2;
echo "2 Month ahead: " 
.date('Y-m-d', 
    mktime(
    	0,
  		0, 
  		0, 
  		date('m',strtotime(date('Y-m-d')))+$nMonths
 	)
) . "\n";


$nYears = 2;
echo "2 Years ahead: "
.date('Y-m-d', 
    mktime(
    	0,
  		0, 
  		0, 
  		date('m',strtotime(date('Y-m-d'))),
  		date('d',strtotime(date('Y-m-d'))),
  		date('Y',strtotime(date('Y-m-d')))+$nYears
 	)
) . "\n";

PHP:

A Ready to use function for php < 5.3 from:
http://de3.php.net/manual/en/function.date-add.php#95574

function add_date($givendate,$day=0,$mth=0,$yr=0) {
    $cd = strtotime($givendate);
    $newdate = date('Y-m-d h:i:s', mktime(date('h',$cd),
    date('i',$cd), date('s',$cd), date('m',$cd)+$mth,
    date('d',$cd)+$day, date('Y',$cd)+$yr));
    return $newdate;
}

// 2 month
echo dateAdd(date('Y-m-d'), 0, 2, 0);

// 2 years
echo dateAdd(date('Y-m-d'), 0, 0, 2);

PHP: