What is expected DATETIME format for Wordpress Codex WP Meta Query parameter?

This docs:

https://codex.wordpress.org/Class_Reference/WP_Meta_Query

Tells you can compare DATETIME values.

What is expected format of DATETIME for WP_Meta_Query?

From the Wordpress documentation, it seems that you should use the following string format: Y-m-d H:i:s.

So a meta query could be:

$meta_query_args = array(
    'relation' => 'OR', // Optional, defaults to "AND"
    array(
        'key'     => 'field_key_with_datetime',
        'value'   => '2018-03-19 08:23:25', // Y-m-d H:i:s format
        'compare' => '=',
        'type'    => 'DATETIME'
    )
);
$meta_query = new WP_Meta_Query( $meta_query_args );

You can use the wordpress current_time function to get the current DATETIME:

$meta_query_args = array(
    'relation' => 'OR',
    array(
        'key'     => 'field_key_with_datetime',
        'value'   => current_time( 'mysql' ), // will return something like '2018-03-19 08:23:25'
        'compare' => '=',
        'type'    => 'DATETIME'
    )
);
$meta_query = new WP_Meta_Query( $meta_query_args );

Source: https://developer.wordpress.org/reference/functions/current_time/