Tests different methods of timezone handling in WordPress.

wp-time-test.php

<?php

class WP_Time_Test extends WP_UnitTestCase {

	public function test_strtotime() {

		$offset = -5;

		update_option( 'gmt_offset', $offset );
		update_option( 'timezone_string', 'America/New_York' );

		// A date relative to the site's timestamp.
		$current_date = current_time( 'mysql' );

		// Convert it to a timestamp.
		$current_time = strtotime( $current_date );

		// Remember, strtotime() uses the default timezone if none is supplied.
		// This means that it just handed us a timestamp for when that datetime
		// occurred in the GMT timezone. We have to correct that to the time
		// that that datetime occurred in our current timezone, by removing the
		// excess based on the our current UTC offset.
		// This will be wrong if DST!
		$current_time -= $offset * HOUR_IN_SECONDS;

		// Then convert the timestamp back to date format.
		$gmt_date_from_current = gmdate( 'Y-m-d H:i:s', $current_time );

		// Now, get the straight GMT date.
		$current_date_gmt = current_time( 'mysql', true );

		// The one that we converted from the site's timezone should match exactly.
		$this->assertSame( $current_date_gmt, $gmt_date_from_current );
	}

	public function test_datetime_object() {

		update_option( 'gmt_offset', -5 );
		update_option( 'timezone_string', 'America/New_York' );

		// A date relative to the site's timezone.
		$current_date = current_time( 'mysql' );

		// Create a datetime object from it, passing in the site's timezone.
		$date = new DateTime(
			$current_date
			, new DateTimeZone( get_option( 'timezone_string' ) )
		);

		// Option 1:

		// Convert it to a timestamp.
		$current_time = $date->getTimestamp();

		// Then convert the timestamp back to date format.
		$gmt_date_from_current = gmdate( 'Y-m-d H:i:s', $current_time );

		// Now, get the straight GMT date.
		$current_date_gmt = current_time( 'mysql', true );

		// The one that we converted from the site's timezone should match exactly.
		$this->assertSame( $current_date_gmt, $gmt_date_from_current );

		// Option 2:

		// If we change the timezone to UTC...
		$date->setTimezone( new DateTimeZone( 'UTC' ) );

		// ...we can also convert it using the format() method.
		$this->assertSame( $current_date_gmt, $date->format( 'Y-m-d H:i:s' ) );
	}
}