Thank you very much for looking into this @Libbum. I have been thinking of work-arounds and one thing I thought was to try and separate them as you suggested.
Another possibility is to let the JS side handle all the display for this element (not changing the input value) attribute from Elm. For this possibility, I would like to send the initial timestamp from Elm to JS, which can then handle the display (and send any updated value back to Elm just to store in the Model). However, I am having some issues sending the initial value (written in a question here.
In short, I think when the Elm sends the initial timestamp, the JS is not yet ready to receive it. If I send it with some time delay after the page loads, things work. But instead of adding an arbitrary time delay (which may or may not work), is there a DOMContentLoaded or DOM ready event on Elm that I can use to send the value? Thank you again for taking to look at this.