al_add_timer_count man page
al_add_timer_count — Allegro 5 API
#include <allegro5/allegro.h> void al_add_timer_count(ALLEGRO_TIMER *timer, int64_t diff)
Add diff to the timer's counter value. This is similar to writing:
al_set_timer_count(timer, al_get_timer_count(timer) + diff);
except that the addition is performed atomically, so no ticks will be lost.
Allegro reference manual