ref: 2fed6efe0a9d1192963c02c6ba88b8b9d0754d12 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; }