Question

I tried some date functions, because I would like to get some dates of the next month with PHP, but I experienced some problems.

The current date as a MySQL-compatible timestamp (date('Y-m-d H:i:s');) is the following:
'2012-05-31 14:59:19'

• date('t', strtotime('next month')); which gives the number of days in the next month, returns 31 as a result… which is not correct, because June consists of only 30 days (seems like it outputs the days of July).
• date('Y-m-d H:i:s', strtotime('next month'));, which gives the next month’s timestamp format, returns '2012-07-01 14:59:19', but I would expect the following:
  '2012-06-30 14:59:19'.

So I tried some codes, here are my experiments, using date(), strtotime() and DateTime class: http://pastebin.com/3iaH4iSZ

And the OUTPUT looks like the following (you can also see it here!):

Are these PHP bugs, or do I misunderstand something?

---

I would expect the same results as when using MySQL’s date functions, just a few examples:

SELECT NOW( )

2012-05-31 17:51:15

SELECT DATE_ADD(NOW(), INTERVAL 1 MONTH)

2012-06-30 17:51:15

SELECT DATE_ADD( '2013-01-30 17:51:15', INTERVAL 1 MONTH )

2013-02-28 17:51:15

SELECT DATE_ADD( '2013-01-31 17:51:15', INTERVAL 1 MONTH )

2013-02-28 17:51:15

Answer 1

OK, I think I found the solution, which behaves similar to MySQL’s DATE_ADD function with INTERNAL 1 MONTH.
I tested my function with all the possibly problematic cases, and it seems to work correctly.
I paste it here, maybe someone will find it useful. I put some comments in this code, so I think it’s clear enough.

The function is called getOneMonthLaterTimestamp. An example:

echo getOneMonthLaterTimestamp('2012-05-31 17:51:15');

Output: 2012-06-30 17:51:15

The function

/**
 * Test function for dumping variables in a readable way
 * 
 * @param mixed $stuff
 * @param string $text
 * @return string 
 */
function my_var_export($stuff, $text = '...') {
    return '<p>' . $text . ' (' . gettype($stuff) . '):</p><pre>' . var_export($stuff, TRUE) . '</pre>';
}

/**
 * Get timestamp format of the date one month later 
 * than the date given in the argument/current date if left empty.
 * 
 * Behaves similar to MySQL's DATE_ADD function with INTERVAL 1 MONTH:
 * SELECT DATE_ADD('2012-05-31 17:51:15', INTERVAL 1 MONTH)
 * --> 2012-06-30 17:51:15
 * SELECT DATE_ADD('2013-01-30 17:51:15', INTERVAL 1 MONTH )
 * --> 2013-02-28 17:51:15
 * 
 * these are equivalent to:
 * echo getOneMonthLaterTimestamp('2012-05-31 17:51:15');
 * echo getOneMonthLaterTimestamp('2013-01-30 17:51:15');
 * 
 * You can also call it without an argument. This way, the current date is taken as a basis.
 * echo getOneMonthLaterTimestamp();
 * 
 * @param string $DateTime_param date/time string
 * @see http://www.php.net/manual/en/datetime.formats.php
 * @see http://www.php.net/manual/en/datetime.construct.php
 * 
 * @return string Date one month later as a MySQL-compatible timestamp format
 */
function getOneMonthLaterTimestamp($DateTime_param = NULL) {
    // if argument is left empty, the current date is taken as a basis
    if (empty($DateTime_param)) {
        $DateTime_param = date('Y-m-d H:i:s');
    }

    $lastDayOfNextMonth = new DateTime($DateTime_param);
    $lastDayOfNextMonth->modify('last day of next month');

    $nextMonth = new DateTime($DateTime_param);
    $nextMonth->modify('next month');

    if ($lastDayOfNextMonth->format('n') < $nextMonth->format('n')) {
        $oneMonthLaterTimestamp = $lastDayOfNextMonth->format('Y-m-d H:i:s');
    }
    else {
        $oneMonthLaterTimestamp = $nextMonth->format('Y-m-d H:i:s');
    }

    return $oneMonthLaterTimestamp;
}

Test cases

$timestamps_to_test_array = array(
  date('Y-m-d H:i:s'),   // 1. current date
  '2011-01-28 23:59:59', // 2.
  '2011-01-29 23:59:59', // 3.
  '2011-01-30 23:59:59', // 4.
  '2011-01-31 23:59:59', // 5.
  '2012-01-28 23:59:59', // 6.
  '2012-01-29 23:59:59', // 7.
  '2012-01-30 23:59:59', // 8.
  '2012-01-31 23:59:59', // 9.
  '2012-02-29 23:59:59', // 10.
  '2012-03-30 23:59:59', // 11.
  '2012-03-31 23:59:59', // 12.
  '2012-04-30 23:59:59', // 13.
  '2012-05-31 23:59:59', // 14.
  '2012-12-31 23:59:59', // 15.
  '2013-01-31 23:59:59', // 16.
  '2013-02-28 23:59:59', // 17.
);

$i = 1;

foreach ($timestamps_to_test_array as $timestamp_to_test) {
    $oneMonthLaterTimestamp = getOneMonthLaterTimestamp($timestamp_to_test);
    echo my_var_export($timestamp_to_test, '[' . $i . ']. Timestamp to test ($timestamp_to_test)');
    echo my_var_export($oneMonthLaterTimestamp, 'Timestamp + 1 month (getOneMonthLaterTimestamp($timestamp_to_test))');
    echo '<hr />';
    $i++;
}

Output of the test cases

I pasted the output here: http://pastebin.com/rY5ZRBs9.
Here’s a screenshot of it: https://i.stack.imgur.com/9q23l.png

Getting the number of days in the next month

/**
 * Get number of days in the next month
 *
 * @param string $DateTime_param date/time string
 * @return int Number of days in the next month
 */
function getNumberOfDaysInNextMonth($DateTime_param = NULL) {
    // if argument is empty, the current date is taken as a basis
    if (empty($DateTime_param)) {
        $DateTime_param = date('Y-m-d H:i:s');
    }

    // DateTime instance
    $dateCurrent = new DateTime($DateTime_param);
    $dateCurrent->modify('last day of next month');
    return (int)$dateCurrent->format('t');
}

/**
 * It's identical to getNumberOfDaysInNextMonth()
 *
 * @see getNumberOfDaysInNextMonth
 */
function getLastDayOfNextMonth($DateTime_param = NULL) {
    return getNumberOfDaysInNextMonth($DateTime_param);
}

Test case

$lastDayOfNextMonth = getLastDayOfNextMonth('2012-05-31 03:50:27');
echo my_var_export($lastDayOfNextMonth, "Get last day of next month (getLastDayOfNextMonth('2012-05-31 03:50:27'))");

Output:

Get last day of next month (getLastDayOfNextMonth('2012-05-31 03:50:27')) (integer):
30