There are hundreds of flash clocks available on the web and it's quit easy to replace one of the clocks in the module with your own clock. This is what you have to do:
Now the clock has been added and it will be displayed on your site if you select the first clock in the backend. If can also change the clock thumbnail in the backend if you want. Just take a snapshot of your clock and save it as <base_dir>/modules/mod_ulti_clocks/preview/clock1.jpg.
An alternative way is to replace the files in the zip-file in stead of on the server and then install the module.