Complete Code.

// Set UI labels for Custom Post Type
$labels = array(
\'name\'                => _x( \'Events\', \'evn_calendar\'),
\'singular_name\'       => _x( \'events\', \'evn_calendar\'),
\'menu_name\'           => __( \'Events\', \'evn_calendar\' ),
\'parent_item_colon\'   => __( \'Parent Event\', \'evn_calendar\' ),
\'all_items\'           => __( \'All Events\', \'evn_calendar\' ),
\'view_item\'           => __( \'View Event\', \'evn_calendar\' ),
\'add_new_item\'        => __( \'Add New Event\', \'evn_calendar\' ),
\'add_new\'             => __( \'Add New\', \'evn_calendar\' ),
\'edit_item\'           => __( \'Edit Event\', \'evn_calendar\' ),
\'update_item\'         => __( \'Update Event\', \'evn_calendar\' ),
\'search_items\'        => __( \'Search Event\', \'evn_calendar\' ),
\'not_found\'           => __( \'Not Found\', \'evn_calendar\' ),
\'not_found_in_trash\'  => __( \'Not found in Trash\', \'evn_calendar\' ),
);

// Set other options for Custom Post Type

$args = array(
\'label\'               => __( \'events\', \'evn_calendar\' ),
\'description\'         => __( \'Event news and reviews\', \'evn_calendar\' ),
\'labels\'              => $labels,
// Features this CPT supports in Post Editor
\'supports\'            => array( \'title\', \'editor\', \'excerpt\', \'author\', \'thumbnail\', \'comments\', \'revisions\', ),
// You can associate this CPT with a taxonomy or custom taxonomy. 
\'taxonomies\'          => array( \'genres\' ),
/* A hierarchical CPT is like Pages and can have
* Parent and child items. A non-hierarchical CPT
* is like Posts.
*/ 
\'hierarchical\'        => false,
\'public\'              => true,
\'show_ui\'             => true,
\'show_in_menu\'        => true,
\'show_in_nav_menus\'   => true,
\'show_in_admin_bar\'   => true,
\'menu_position\'       => 20,
\'can_export\'          => true,
\'has_archive\'         => true,
\'exclude_from_search\' => false,
\'publicly_queryable\'  => true,
\'capability_type\'     => \'post\',
);

// Registering your Custom Post Type
register_post_type( \'events\', $args );



function eventcalendar_add_meta_box() {
$screens = array( \'events\' );
foreach ( $screens as $screen ) {
add_meta_box(
\'Event Information\',
\'Event Information\',
\'eventcalendar_show_custom_meta_box\',
$screen,
\'normal\',
\'high\'
);
}
}
add_action( \'add_meta_boxes\', \'eventcalendar_add_meta_box\' );
function eventcalendar_show_custom_meta_box( $post ) {


wp_nonce_field( \'evn_from_date\', \'evn_from_date_nonce\' );
$from_date = get_post_meta( $post->ID, \'_evn_from_date\', true );
echo \'<label for="evn_from_date">\'; _e( \'When\', \'event-calendar-plugin\' ); echo \'</label> \';
echo \'<input type="date" id="evn_from_date" name="evn_from_date" value="\' . esc_attr( $from_date ) . \'" size="25" />\';

wp_nonce_field( \'evn_from_time\', \'evn_from_time_nonce\' );
$from_time = get_post_meta( $post->ID, \'_evn_from_time\', true );
echo \'<input type="time" id="evn_from_time" name="evn_from_time" value="\' . esc_attr( $from_time ) . \'" size="25" />\';


wp_nonce_field( \'evn_to_date\', \'evn_to_date_nonce\' );
$from_date = get_post_meta( $post->ID, \'_evn_to_date\', true );
echo \'<label for="evn_to_date">\'; _e( \'To\', \'event-calendar-plugin\' ); echo \'</label> \';
echo \'<input type="date" id="evn_to_date" name="evn_to_date" value="\' . esc_attr( $from_date ) . \'" size="25" />\';

wp_nonce_field( \'evn_to_time\', \'evn_to_time_nonce\' );
$from_time = get_post_meta( $post->ID, \'_evn_to_time\', true );
echo \'<input type="time" id="evn_to_time" name="evn_to_time" value="\' . esc_attr( $from_time ) . \'" size="25" />\';



$request  = wp_safe_remote_get(plugins_url(\'/js/latest.json\', __FILE__));                  
$data = wp_remote_retrieve_body( $request );
$data = json_decode($data);

wp_nonce_field( \'evn_timezone\', \'evn_timezone_nonce\' );
$evn_timezone = get_post_meta( $post->ID, \'_evn_timezone\', true );


echo \'<select id="evn_timezone" name="evn_timezone" >\';

foreach ($data->countries as $country) {

$selected = ( $country[ $country->name ] == esc_attr( $country->name ) ) ? \' selected="selected"\' : \'\';

printf( \'<option value="%s(%s)"%s>%s</option>\', $country->abbr, $country->name, $selected, $country->name );
}

echo \'</select>\';


wp_nonce_field( \'evn_all_day\', \'evn_all_day_nonce\' );
$evn_all_day = get_post_meta( $post->ID, \'evn_all_day\', true );
echo \'<input type="checkbox" id="evn_all_day" name="evn_all_day" value="\' . esc_attr( $evn_all_day ) . \'"  />\';
echo \'<label for="evn_all_day">\'; _e( \'All day?\', \'event-calendar-plugin\' ); echo \'</label> \';



wp_nonce_field( \'evn_location\', \'evn_location_nonce\' );
$evn_location = get_post_meta( $post->ID, \'_evn_location\', true );
echo \'<br><label for="evn_location">\'; _e( \'Event Location\', \'event-calendar-plugin\' ); echo \'</label> \';
echo \'<input type="text" id="evn_location" name="evn_location" value="\' . esc_attr( $evn_location ) . \'" size="100" />\';


wp_nonce_field( \'evn_link\', \'evn_link_nonce\' );
$evn_link = get_post_meta( $post->ID, \'_evn_link\', true );
echo \'<br><label for="evn_link">\'; _e( \'Link\', \'event-calendar-plugin\' ); echo \'</label> \';
echo \'<input type="text" id="evn_link" name="evn_link" value="\' . esc_attr( $evn_link ) . \'" size="100" placeholder=""/>\';

}
function eventcalendar_save_meta_box_data( $post_id ) {
if ( ! isset( $_POST[\'evn_from_date_nonce\'] ) && 
! isset( $_POST[\'evn_from_time_nonce\'] )  &&  
! isset( $_POST[\'evn_to_date_nonce\'] ) && 
! isset( $_POST[\'evn_to_time_nonce\'] )  &&  
! isset( $_POST[\'evn_location_nonce\'] ) && 
! isset( $_POST[\'evn_timezone\'] ) && 
! isset( $_POST[\'evn_link\'] ))

{
return;
}
if (! wp_verify_nonce( $_POST[\'evn_from_date_nonce\'], \'evn_from_date\' ) && 
! wp_verify_nonce( $_POST[\'evn_from_time_nonce\'], \'evn_from_time\' ) && 
! wp_verify_nonce( $_POST[\'evn_to_date_nonce\'], \'evn_to_date\' ) && 
! wp_verify_nonce( $_POST[\'evn_to_time_nonce\'], \'evn_to_time\' ) && 

! wp_verify_nonce( $_POST[\'evn_timezone_nonce\'], \'evn_timezone\' ) && 

! wp_verify_nonce( $_POST[\'evn_location_nonce\'], \'evn_location\' ) &&
! wp_verify_nonce( $_POST[\'evn_link_nonce\'], \'evn_link\' )  ) {
return;
}
if ( defined( \'DOING_AUTOSAVE\' ) && DOING_AUTOSAVE ) {
return;
}
if ( isset( $_POST[\'post_type\'] ) && \'page\' == $_POST[\'post_type\'] ) {
if ( ! current_user_can( \'edit_page\', $post_id ) ) {
return;
}
} else {
if ( ! current_user_can( \'edit_post\', $post_id ) ) {
return;
}
}
if ( ! isset( $_POST[\'evn_from_date\'] ) && 
! isset( $_POST[\'evn_from_time\'] ) && 
! isset( $_POST[\'evn_to_date\'] ) && 
! isset( $_POST[\'evn_to_time\'] ) &&
! isset( $_POST[\'evn_timezone\'] ) &&  

! isset( $_POST[\'evn_location\'] )&&
! isset( $_POST[\'evn_link\'] ) 
) {
return;
}

$evn_from_date = sanitize_text_field( $_POST[\'evn_from_date\'] );
update_post_meta( $post_id, \'_evn_from_date\', $evn_from_date );

$evn_from_time = sanitize_text_field( $_POST[\'evn_from_time\'] );
update_post_meta( $post_id, \'_evn_from_time\', $evn_from_time );

$evn_to_date = sanitize_text_field( $_POST[\'evn_to_date\'] );
update_post_meta( $post_id, \'_evn_to_date\', $evn_to_date );

$evn_to_time = sanitize_text_field( $_POST[\'evn_to_time\'] );
update_post_meta( $post_id, \'_evn_to_time\', $evn_to_time );

$evn_timezone = sanitize_text_field( $_POST[\'evn_timezone\'] );
update_post_meta( $post_id, \'_evn_timezone\', $evn_timezone );

$evn_link = sanitize_text_field( $_POST[\'evn_link\'] );
update_post_meta( $post_id, \'_evn_link\', $evn_link );

$evn_location = sanitize_text_field( $_POST[\'evn_location\'] );
update_post_meta( $post_id, \'_evn_location\', $evn_location );
}
add_action( \'save_post\', \'eventcalendar_save_meta_box_data\' );