Demonstrate impact of date_default_timezone_set()

test-date-default-timezone-set.php

<?php

$date_string = '2015-11-13 09:59:00';

// Normal WordPress Way
date_default_timezone_set( 'UTC' );
$date_utc = new DateTime( $date_string );

// Rare Occassions
date_default_timezone_set( 'America/Los_Angeles' );
$date_pdt = new DateTime( $date_string );

if ( $date_utc->format( 'U' ) === $date_utc->format( 'U' ) ) {
	error_log( 'Horray! The timestamp of our date is the same, because they are both UTC.' );
} else {
	error_log( 'Oh no! The timestamp of our date is different, because it was not UTC.' );
}

// Normal WordPress Way
date_default_timezone_set( 'UTC' );
$timestamp_utc = strtotime( $date_string );

// Rare Occassions
date_default_timezone_set( 'America/Los_Angeles' );
$timestamp_pdt = strtotime( $date_string );

if ( $timestamp_utc === $timestamp_pdt ) {
	error_log( 'Horray! The timestamp of our date is the same, because they are both UTC.' );
} else {
	error_log( 'Oh no! The timestamp of our date is different, because it was not UTC.' );
}