ref: 07c8c2c838256ea0b3b9dd6d5a70ba32ff3bafab 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; }