程序代码：

#include <stdio.h>

int main()
{
    int hour, minu, sec;
    printf("请输入时间，格式 时:分:秒\n");
    scanf("%d:%d:%d",&hour,&minu,&sec);

    sec += 30;
    if (sec >= 60)
    {
        minu++;
        sec -= 60;
    }
    if (minu >= 60)
    {
        hour++;
        minu -= 60;
    }
    if (hour >= 24)
    {
        hour = 0;
    }
    printf("后30秒为 ");
    printf("%d:%d:%d\n", hour, minu, sec);
    return 0;
}