Comparing hexadecimal numbers with dates, you can see it’s possible to use a 6-digits hexadecimal number representing a day. The year fits in 3 digits (<=4.096); month fits in 1 digit (16) and day in 2 (256, but we need only 31). It’s the same as a HTML color!

For example: today, 23th august. 2021 in hexadecimal is 7E5; august is 8; 23 in 16-base is 17. So, the color for today is #7E5817.

Thinking about, I made a PHP function 10 years ago. It returns a color for a given day. It’s like:

function daycolor($date) {
    $dts = strtotime($date);
    $y = dechex(date("Y", $dts));
    $m = dechex(date("n", $dts));
    $d = dechex(date("d", $dts));
    if (strlen($d) < 2) {
        $d = "0" . $d;
    }
    $dc = "#$y$m$d";
    return $dc;
}

It’s a very simple code, easy to convert to any other language. Unfortunately, the colors come too similar, changing overly slow..