ref: 922fc6faa00ab4e0f46b9f3d3c6bbd25a1c2201d dir: /sys/src/ape/lib/ap/gen/difftime.c/
#include <time.h> /* Difference in seconds between two calendar times */ double difftime(time_t t1, time_t t0) { return (double)t1-(double)t0; }