Countdown Timer от lastlegume

Simply click on the icon on your addons taskbar, enter a date, and watch as the extension counts down until the start of that day. You may either name a timer or leave its name blank, in which case the timer will be identified solely by its end date. The toolbar icon represents the time elapsed since the start of the countdown in a simple pie chart. You can add as many dates as you want and choose between all of them. Timers are deleted one hour after they finish to reduce clutter.

Note that when there are no timers, the extensions defaults to counting down until the next year. This is not a bug. It will also clear any timers that are dates in the past automatically, as counting down to a date that has already happened does not make sense.
Both a light and dark modes are present in the extension popup.