Request for Comments: DateTime and Daylight Saving Time Transitions

• Version: 0.1
• Date: 2011-07-20
• Author: Daniel Convissor danielc@php.net
• Status: Draft
• First Published at: https://wiki.php.net/rfc/datetime_and_daylight_saving_time

PHP's DateTime class has unexpected outcomes when dealing with the transitions between Daylight Saving Time and Standard Time.

Properly defining, documenting and unit testing DateTime's behaviors is important for PHP's future. This document seeks agreement on what the expected behaviors should be.

Getting these issues straightened out before 5.4 goes into Release Candidate status seems wise.

The America/New_York time zone will be used as the basis for this discussion. The Standard/Daylight transition in this zone happens at 2:00 am. Other zones use different transition times.

01:59:59 Standard Time
+ 1 second
= 03:00:00 Daylight Saving Time (United States)

• The hour of 2am gets skipped.
• There are 23 hours on that Sunday.
• 2010-03-14

1:59:59 Daylight Saving Time (United States)
+ 1 second
= 1:00:00 Standard Time

• The hour of 1am gets replayed.
• There are 25 hours on that Sunday.
• 2010-11-07

Times between 2:00:00 am and 2:59:59 am do not exist during the Spring transition. Attempts to create such times should be rounded forward, like PHP currently does when trying to create February 29th on a non-leap year.

date_default_timezone_set('America/New_York');
$date = new DateTime('2010-03-14 02:30:00');
echo $date->format('Y-m-d H:i:s T') . "\n";
// Expected: 2010-03-14 03:30:00 EDT
// Actual:   2010-03-14 02:30:00 EDT

PHP needs a means to create objects representing times during the hour repeated on the Daylight/Standard transition day (between 1:00:00 am and 1:59:59 am Standard Time).

This RFC proposes adding a third parameter to DateTime::__construct(), bool $is_daylight_saving_time. The default value would be NULL. When NULL, PHP will use the current behavior, which uses the DST/ST in place at the specified time and uses DST during the repeated hour in the fall.

Force Daylight Saving Time:

date_default_timezone_set('America/New_York');
$date = new DateTime('2010-11-07 01:30:00', null, true);
echo $date->format('Y-m-d H:i:s T') . "\n";
// Expected: 2010-11-07 01:30:00 EDT

Force Standard Time:

date_default_timezone_set('America/New_York');
$date = new DateTime('2010-11-07 01:30:00', null, false);
echo $date->format('Y-m-d H:i:s T') . "\n";
// Expected: 2010-11-07 01:30:00 EST

Default to Daylight Time during repeated hour, PHP's current behavior:

date_default_timezone_set('America/New_York');
$date = new DateTime('2010-11-07 01:30:00');
echo $date->format('Y-m-d H:i:s T') . "\n";
// Expected & Actual: 2010-11-07 01:30:00 EDT

Attempts to create a Standard Time during Daylight Saving Time should be rolled forward to DST. Attempts to create a Daylight Saving Time during Standard Time should be rolled back to ST.

date_default_timezone_set('America/New_York');
// Saturday, 23.5 hrs before DST transition
$date = new DateTime('2010-03-13 02:30:00');
$date->add(new DateInterval('P1D'));
// Sunday, 2:30 doesn't exist, roll forward 1 hr
echo $date->format('Y-m-d H:i:s T') . "\n";
// Expected & Actual: 2010-03-14 03:30:00 EDT  

If you know of other problems, please add them here or bring them to the attention of the authors.