note 102705 modified in function.strtotime by danbrown

PHP strtotime() does not handle the ISO8601 notation for "duration."  This handles the most likely subset of the duration standard (the one that is used by eBay).
<?php // RAY_duration_strtotime.php
error_reporting(E_ALL);
date_default_timezone_set('America/New_York');

// WHAT DOES THIS STRING MEAN?
$str = 'P1Y3M2DT16H2M40S';

// IT IS THE ISO8601 FORMAT FOR A DURATION.
// SEE http://en.wikipedia.org/wiki/ISO_8601

// IT MEANS THIS:
// P   = Pending (INDICATOR OF ISO8601 DURATION)
// 1Y  = 1 Year
// 3M  = 3 Months
// 2D  = Two Days
// T   = Time Separator (M is ambiguous)
// 16H = Sixteen hours
// 2M  = Two minutes
// 40S = Forty seconds

// STRTOTIME CANNOT HANDLE ISO-8601 DURATIONS
$dur = strtotime($str);
var_dump($dur);
echo "<br/>";

// GET THE DURATION IN SECONDS FOR USE IN ARITHMETIC
$dur = duration_strtotime($str);
echo "<br/>" . number_format($dur) . ' SECONDS';

// USE THE DURATION IN ARITHMETIC
echo "<br/>" . date('r', strtotime("NOW + $dur SECONDS"));

// A FUNCTION TO MAKE A DURATION STRING INTO A NUMBER OF SECONDS
function duration_strtotime($str, $sho=FALSE)
{
    // REMOVE THE AMBIGUITY OF MONTH AND MINUTE -- MAKE MONTH = X
    $arr = explode('T', $str);
    $arr[0] = str_replace('M', 'X', $arr[0]);
    $new = implode('T', $arr);

    // EXPAND THE STRING INTO SOMETHING SENSIBLE
    $new = str_replace('S', 'seconds ', $new);
    $new = str_replace('M', 'minutes ', $new);
    $new = str_replace('H', 'hours ',   $new);
    $new = str_replace('T', ' ',        $new);
    $new = str_replace('D', 'days',     $new);
    $new = str_replace('X', 'months ',  $new);
    $new = str_replace('Y', 'years ',   $new);
    $new = str_replace('P', NULL,       $new);

    if ($sho) var_dump($new);

    // RETURN THE NUMBER OF SECONDS IN THE DURATION
    return strtotime($new) - strtotime('NOW');
}
?>

--was--
PHP strtotime() does not handle the ISO8601 notation for "duration."  This handles the most likely subset of the duration standard (the one that is used by eBay).
<?php // RAY_duration_strtotime.php
error_reporting(E_ALL);
date_default_timezone_set('America/New_York');

// WHAT DOES THIS STRING MEAN?
$str = 'P1Y3M2DT16H2M40S';

// IT IS THE ISO8601 FORMAT FOR A DURATION.
// SEE http://en.wikipedia.org/wiki/ISO_8601

// IT MEANS THIS:
// P   = Pending (INDICATOR OF ISO8601 DURATION)
// 1Y  = 1 Year
// 3M  = 3 Months
// 2D  = Two Days
// T   = Time Separator (M is ambiguous)
// 16H = Sixteen hours
// 2M  = Two minutes
// 40S = Forty seconds

// STRTOTIME CANNOT HANDLE ISO-8601 DURATIONS
$dur = strtotime($str);
var_dump($dur);
echo "<br/>";

// GET THE DURATION IN SECONDS FOR USE IN ARITHMETIC
$dur = duration_strtotime($str);
echo "<br/>" . number_format($dur) . ' SECONDS';

// USE THE DURATION IN ARITHMETIC
echo "<br/>" . date('r', strtotime("NOW + $dur SECONDS"));

// A FUNCTION TO MAKE A DURATION STRING INTO A NUMBER OF SECONDS
function duration_strtotime($str, $sho=FALSE)
{
    // REMOVE THE AMBIGUITY OF MONTH AND MINUTE -- MAKE MONTH = X
    $arr = explode('T', $str);
    $arr[0] = str_replace('M', 'X', $arr[0]);
    $new = implode('T', $arr);

    // EXPAND THE STRING INTO SOMETHING SENSIBLE
    $new = str_replace('S', 'seconds ', $new);
    $new = str_replace('M', 'minutes ', $new);
    $new = str_replace('H', 'hours ',   $new);
    $new = str_replace('T', ' ',        $new);
    $new = str_replace('D', 'days',     $new);
    $new = str_replace('X', 'months ',  $new);
    $new = str_replace('Y', 'years ',   $new);
    $new = str_replace('P', NULL,       $new);

    if ($sho) var_dump($new);

    // RETURN THE NUMBER OF SECONDS IN THE DURATION
    return strtotime($new) - strtotime('NOW');
}

http://php.net/manual/en/function.strtotime.php