Just thought I would share how I put the date below the time for the default Clock module using only CSS - no need to modify or create your own clock module!

custom.css

/* 
    CLOCK
*/

#module_2_clock > div > div {
    display: flex;
    flex-direction: column;
}

/* Date */
#module_2_clock > div > div > div.date {
    order: 2;
}

/* Time */
#module_2_clock > div > div > div.time {
    order: 1;

    font-family: Roboto, sans-serif;
    font-weight: 300;
    font-size: 80px;
    line-height: 75px;
    letter-spacing: -3px;
    color: #fff;    
}

Also, to show the week on a separate line and make it a little bit smaller, use HTML tags within brackets when setting the date format, like this:

config.js

{
  module: "clock",
  position: "top_right",
  config: {
    displaySeconds: false,
    dateFormat: "dddd D MMMM[< br/ >< small >vecka] W[< /small >]",
  }
},

NOTE: Since the forum is sanitising HTML, remove all the spaces from the tags above in < small >, < /small > and < br/ > to make it work.

Hope this can inspire and help someone! :)