Hey all! I put together a quick video on how to create a decrementing timer die in Roll20 using one API script and 2 macros. The whole process is super simple, requires no knowledge of scripting or anything. The only limitation is that it requires the use of the API, which means you have to be a top-level subscriber. Below is the video, and in the description of that video is a link to the timer images I used.
Hope this is useful!