PHP: Adding months to a date, while not exceeding the last day of the month

Looking to create a function that will do this in PHP.

I need to add a number of months to a date, but not exceed the last day of the month in doing so.

For example:

Add 1 month to January (1-28th), 2011, should produce February (1-28th), 2011.
Add 1 month to January 30th, 2011, should produce February 28th, 2011.
Add 3 months to January 31st, 2011, should produce April 30th, 2011.
Add 13 months to January 30th, 2011, should produce February 29th, 2012.
Add 1 month to October 31st, 2011, should produce November 30th, 2011.

If I use date addition in PHP, I get overruns:

Adding 1 month to January 30th, 2011, results in March 2nd, 2011.

My specification doesn’t allow me to overrun into a new month.

What’s the easiest method to accomplish this?

Answer

You can compare the day of the month before and after you add 1 month. If it’s not the same, you exceeded the next month.

function add($date_str, $months)
{
    $date = new DateTime($date_str);

    // We extract the day of the month as $start_day
    $start_day = $date->format('j');

    // We add 1 month to the given date
    $date->modify("+{$months} month");

    // We extract the day of the month again so we can compare
    $end_day = $date->format('j');

    if ($start_day != $end_day)
    {
        // The day of the month isn't the same anymore, so we correct the date
        $date->modify('last day of last month');
    }

    return $date;
}

$result = add('2011-01-28', 1);   // 2011-02-28
$result = add('2011-01-31', 3);   // 2011-04-30
$result = add('2011-01-30', 13);  // 2012-02-29
$result = add('2011-10-31', 1);   // 2011-11-30
$result = add('2011-12-30', 1);   // 2011-02-28