Question

I am seeing very strange behaviour, with DateTime choosing to add a day sometimes, but not others.

<?php
// If you're running this after Jan 2012, use: new DateTime(date('Y-m-d', strtotime('2012-01-09')));
$month_end_date = new DateTime();
$month_end_date->modify('last day of this month');

$event_end_date = new DateTime('2012-03-15');

if ($event_end_date > $month_end_date) {
  // Using this line a day is never added on below and the date stays as 31 Jan 2012
  $event_end_date = clone $month_end_date;
  // This line allows the ->add() call to work, and gives 1 Feb 2012 as output:
  #$event_end_date = new DateTime($month_end_date->format('Y-m-d'));
}

$event_end_date->add(new DateInterval('P1D'));

// Date should now be 1st Feb
echo "Should be 1 Feb:   ". $event_end_date->format('Y-m-d');
?>

It appears to be the ->modify('last day of this month') line which breaks my code; it will print 1 Feb 2012 if I replace the first two lines with $month_end_date = new DateTime('2011-01-31'); or

$month_end_date = new DateTime('last day of this month'); 
$month_end_date = new DateTime($month_end_date->format(DateTime::W3C)); 

or use my alternative $event_end_date = new DateTime($month_end_date->format('Y-m-d'));.

Does it make sense that I need to call format before making a second modification?

Answer 1

It appears the usage of “first” or “last” directly in the constructor of the DateTime object causes it to become immutable. This does seem like a bug.

e.g.

date_default_timezone_set('Europe/London');
ini_set('display_errors', 1);
error_reporting(E_ALL);

$interval = new DateInterval('P1D');

$date1 = new DateTime('last day of this month');
var_dump($date1);
$date1->add($interval);
var_dump($date1);

echo "-------------------------------\n\n";

$lastDayOfMonth = date('Y-m-d H:i:s', strtotime('last day of this month'));
$date2 = new DateTime($lastDayOfMonth);
var_dump($date2);
$date2->add($interval);
var_dump($date2);

echo "-------------------------------\n\n";

$date3 = new DateTime('2012-01-31 '.date('H:i:s'));
var_dump($date3);
$date3->add($interval);
var_dump($date3);

Results in:

object(DateTime)[2]
  public 'date' => string '2012-01-31 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)

object(DateTime)[2]
  public 'date' => string '2012-01-31 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)

-------------------------------

object(DateTime)[3]
  public 'date' => string '2012-01-31 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)

object(DateTime)[3]
  public 'date' => string '2012-02-01 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)

-------------------------------

object(DateTime)[4]
  public 'date' => string '2012-01-31 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)

object(DateTime)[4]
  public 'date' => string '2012-02-01 11:40:10' (length=19)
  public 'timezone_type' => int 3
  public 'timezone' => string 'Europe/London' (length=13)