Is It Legal To Use Element ID To Pass Data

I wrote a little live clock in JS (using Steve Levithan's "date format" code) that formats the time display according to a format string. I then find all elements by classname and search for a classname of "clock" and write the clock string to the innerHTML of every element I find.But recently I added another "feature". Rather than hard code a format string, I instead pass it in ID. Example:

<span class="clock" id="hh:mm:mm tt"></span>

..and the JS:

var doClock = function () {
var now = new Date();
var e = getElementsByClassName('clock');

[code]...

Note that I am passing the clock FORMAT STRING by using the span's ID. It works great...