The main difference being the static prompt will show the time when the prompt was loaded. If I let it sit for a few minutes, then enter a command, that's when I want another timestamp to be added. My goal is to be able to easily see the time difference taken to execute commands, and how long a prompt sat idle. The problem with the continuous clock answer, aside from the fact that I want to use bash, is that it would lose the start timestamp because it would refresh the whole thing.
For example, I would like my prompt to look like this when I open a new shell:
10:30:21 jeff ~ $
Then let's say it just sits there for a minute before I actually finish typing a command and hit Enter
10:30:21 jeff ~ 10:31:28 $ ./long_running.sh 10:36:52 jeff ~ $
Notice it appends another timestamp showing when I actually executed the command. And I can easily see that the command took about 5 minutes to run by subtracting from the start timestamp on the next prompt.