#include <stdio.h>

#define F_CPU 16000000UL
#define CLK_DIV 256.
#define TIME_PER_CYCLE (CLK_DIV/F_CPU)
#define MS(x) ((1/(1000.*TIME_PER_CYCLE))*x)
#define US(x) ((1/(1000000.*TIME_PER_CYCLE))*x)

int main(void) {
        printf("out = %d %f\n", MS(10), MS(10));
}

i get 

out = 0 0.000000

if i use

int main(void) {
        printf("out = %f %f\n", MS(10), MS(10));
}

i get 

out = 625.000000 625.000000

wtf?