How can I display a desired date into a Wordpress page?

Question

I'm trying to find a way to choose a date and display it into a Wordpress page. Simply put, that's the way this may operate:

1. One or several pages are input a shortcode or a PHP code via PHP insert
2. An operator/webmaster chooses a date from let's say a calendar or drop down
3. This date (formatted) shows into the pages that have the shortcode/PHP code

I do not want automatic or current date which I can do with this plugin. Can anybody help achieve this with a plugin or a PHP snippet?

Answer 1

Display a custom date field in Settings->Reading:

function wpse_52414489_custom_date() {
// Add the section to reading settings so we can add our
// fields to it
 add_settings_section(
    'eg_custom_settings',
    'My custom settings',
    '',
    'reading'
 );

// Add the field with the names and function to use for our new
// settings, put it in our new section
 add_settings_field(
    'eg_custom_date',
    'My custom date',
    'eg_custom_date_callback',
    'reading',
    'eg_custom_settings'
 );

// Register our setting so that $_POST handling is done for us and
// our callback function just has to echo the <input>
 register_setting( 'reading', 'eg_custom_date' );
} 

function eg_custom_date_callback() {
 echo '<input name="eg_custom_date" id="eg_custom_date" type="date" value="' . get_option( 'eg_custom_date' ) . '" class="code" /> Explanation text';
}

add_action( 'admin_init', 'wpse_52414489_custom_date' );

Add your shortcode (EDIT: custom date format):

add_filter( 'init', function() {

 add_shortcode( 'my-custom-date', function() {

    return date( 'd/m/Y', strtotime( get_option( 'eg_custom_date' ) ) );

 });

});

Usage:

[my-custom-date]

Output:

2018-09-18