ref: 26f0aa33d91442fc7a56fc5f5c9b56b5f5f84ce2 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; }