Calling
org-timer-set-timer from an Org-mode buffer runs a
countdown timer. Use <;> from agenda buffers, <C-c C-x
;> everwhere else.
org-timer-set-timer prompts the user for a
duration and displays a countdown timer in the modeline.
org-timer-default-timer sets the default countdown
value. Giving a prefix numeric argument overrides this default
value.