Why does iterating over weeks go wrong with PHP date? - php

Question

I'm writing a php script that iterates over the Monday of each week.
However the script seemed to get out of sync after 22nd of October.
<?php
$october_8th = strtotime("2012-10-08");
$one_week = 7 * 24 * 60 * 60;
$october_15th = $october_8th + $one_week;
$october_22nd = $october_15th + $one_week;
$october_29th = $october_22nd + $one_week;
$november_5th = $october_29th + $one_week;
echo date("Y-m-d -> l", $october_8th) . '<br />';
echo date("Y-m-d -> l", $october_15th) . '<br />';
echo date("Y-m-d -> l", $october_22nd) . '<br />';
echo date("Y-m-d -> l", $october_29th) . '<br />';
echo date("Y-m-d -> l", $november_5th) . '<br />';
This would output:
2012-10-08 -> Monday
2012-10-15 -> Monday
2012-10-22 -> Monday
2012-10-28 -> Sunday
2012-11-04 -> Sunday
I would expect it to say the 29th of October but it gets stuck at the 28th.
How should I get around this problem?

Answer 1

A preferred choice would be to use PHP's date-related classes to get the dates.
These classes importantly handle the daylight-savings boundaries for you, in a way that manually adding a given number of seconds to a Unix timestamp (the number from strtotime() that you used) cannot.
The following example takes your start dates and loops four times, each time adding a week to the date.
$start_date = new DateTime('2012-10-08');
$interval = new DateInterval('P1W');
$recurrences = 4;
foreach (new DatePeriod($start_date, $interval, $recurrences) as $date) {
echo $date->format('Y-m-d -> l') . '<br/>';
}
PHP Manual links:
The DatePeriod class
The DateInterval class
The DateTime class

Answer 2

While writing this question I discovered that day light saving time ends at the 28th of October.
Because the date at initialization doesn't contain a specific time automatically midnight is assigned. This however yields a problem when summertime ends. Suddenly the time isn't midnight anymore but one hour before that AND thus a day earlier then you would expect.
An easy fix would be to initialize the time to be midday instead of midnight:
$october_8th = strtotime("2012-10-08 12:00");
Perhaps there might be more elegant solution (you're welcome to leave one), but this will do for this purpose.