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 ..