Clock app timers not showing in Notification Center on Mac mini

The clock app itself does not show a representation of each timer set. Why? I just had a timer go off. The banner quickly appeared and then went away. Going to date to activate the Notification Center did not display the missing timer notification. The alarm could not be turned off short of going to the Activity monitor and killing the Notification Center. Do I have some sort of settings issue? I like using the timer to set time limits on my tasks, and sometimes have several running. But it is really inconvenient to have to kill the Notification Center process in order to turn it off.

[Re-Titled by Moderator]

Original Title: Clock app and timers