\WP_Stager_Integration\Calendar::calculate_offset_timestamp(int $timestamp, string $offset): int

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/**
 * Add an ISO-8601 <code>DateInterval</code> spec offset to a local instant safely (DST aware).
 *
 * Make sure to add a minus symbol <code>'-'</code> before the offset to make it negative, e.g. <code>'-P1D'</code>.
 *
 * @param int       $timestamp  The timestamp
 * @param string    $offset     The offset to apply. Examples: <code>'P1D'</code>, <code>'P1W'</code>, <code>'P1M'</code>
 *
 * @return int  The offset timestamp
 */
public static function calculate_offset_timestamp( $timestamp, $offset ) {

    // Convert to datetime
    $date_time = self::get_datetime_from_timestamp( $timestamp );

    // Check if the offset was passed as a negative offset
    if ( substr( $offset, 0, 1 ) === '-' ) {

        // Remove '-' from the offset
        $offset = str_replace( '-', '', $offset );

        // Subtract interval
        $date_time = $date_time->sub( new \DateInterval( $offset ) );
    } else {

        // Add interval
        $date_time = $date_time->add( new \DateInterval( $offset ) );
    }


    // Return timestamp
    return $date_time->getTimestamp();
}