itimer time value does not decrement after resuming

Question

I'm trying to make a timer that can be paused/resumed with itimer.

I found this stackoverflow page with a suggested method. I've read the linux man page, and I've also tried using getitimer to save the timer, though it still does not work.

In the below code sample, I get 5000, 4537, 4537 and 4537 for d1, d2, d3 and d4, and I am attempting to get something like 5000, 4537, 4536, 4000.

Is there a way I can make it so that the timer value decreases after resuming?

Thank you for suggestions,

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/time.h>
#include <string.h>



struct itimerval old_timer;

void resume(){
    setitimer(ITIMER_REAL, &old_timer, NULL);
}

void pause(){
    struct itimerval zero_timer = { 0 };
    setitimer(ITIMER_REAL, &zero_timer, &old_timer);
}


void timer_handler(){
    puts("RING RING");
}

int main(){
    struct sigaction sa;
    memset(&sa, 0, sizeof (sa));
    sa.sa_handler = timer_handler;
    sigaction(SIGALRM, &sa, NULL);

    old_timer.it_interval.tv_usec = 5000; 
    old_timer.it_interval.tv_sec = 0;
    old_timer.it_value.tv_usec = 5000;
    old_timer.it_value.tv_sec = 0;
    
    resume();
    long int d1 = old_timer.it_value.tv_usec;
    puts("delay1");

    pause();
    long int d2 = old_timer.it_value.tv_usec;
    for(int i=0; i<10; i++){ puts("delay2"); }


    resume();
    long int d3 = old_timer.it_value.tv_usec;
    for(int i=0; i<10; i++){ puts("delay3"); }

    long int d4 = old_timer.it_value.tv_usec;
    for(int i=0; i<10; i++){ puts("delay4"); }

    printf("%ld\n",d1);
    printf("%ld\n",d2);
    printf("%ld\n",d3);
    printf("%ld\n",d4);
}