Ensuring Unix time in PHP - php

Question

My code assumes that zero represents the beginning of the Unix epoch, 1970-01-01 00:00. I upgraded an installation of PHP and now, all of a sudden, zero represents 1970-01-01 01:00 (as verified with date('Y-m-d H:i', 0)). So apparently there is a time zone matter. I put the same code into a sandbox and got 1969-12-31 16:00. I have several unit tests that are broken as a result of this. Time zones do not and should not come into play here.
How can I ensure that date-time functions such as date() always converts zero to 1970-01-01 00:00 regardless of the time zone setting on the particular installation?

Answer 1

Using gmdate() you'll always get 1970-01-01 00:00 for 0, no matter what timezone your server is in:
<?php
date_default_timezone_set('Europe/Berlin');
echo "Europe/Berlin:\n";
echo "gmdate: ".gmdate('d.m.y H:i', 0) . "\n";
echo "date: ".date('d.m.y H:i', 0) . "\n";
date_default_timezone_set('America/Los_Angeles');
echo "\nAmerica/Los_Angeles:\n";
echo "gmdate: ".gmdate('d.m.y H:i', 0) . "\n";
echo "date: ".date('d.m.y H:i', 0) . "\n";
/* OUTPUT:
Europe/Berlin:
gmdate: 01.01.70 00:00
date: 01.01.70 01:00
America/Los_Angeles:
gmdate: 01.01.70 00:00
date: 31.12.69 16:00
*/
https://3v4l.org/FechC

Answer 2

You need to set the default time zone to GMT if you want to use date() like that. For example:
date_default_timezone_set('GMT');
echo date('Y-m-d H:i [I] [e] [O]',0);
The above will show (no matter what the server has been set to):
1970-01-01 00:00 [0] [GMT] [+0000]
Without the date_default_timezone_set('GMT'), or even set to Europe/London, you will get a different result at different times of the year.
From the PHP manual;
date — Format a local time/date
gmdate — Format a GMT/UTC date/time

Answer 3

The solution is to get the timezone setting, set it aside, change the timezone to UTC, perform the calculations, and reset the timezone to its original setting.
So if my original function looked like this:
public function format($argument = null)
{
// Perform some calculations involving date() and strtotime().
return $result;
}
Now it looks like this:
public function format($argument = null)
{
$timezone = date_default_timezone_get();
date_default_timezone_set('UTC');
$result = ...; // Perform some calculations involving date() and strtotime().
date_default_timezone_set($timezone);
return $result;
}