A countdown timer that minimises to tray?
Have a look at Alarm Clock. It is an alarm clock and a timer and minimise to tray.
Installation: sudo apt install alarm-clock-applet
I'm pretty sure that is not what the OP expects, but anyways I like it:
The nerd solution
nohup sleep 60 && notify-send Timer "Time up" && paplay /usr/share/sounds/freedesktop/stereo/complete.oga &
I have to add a way to notify how much time is left ..