I finally took the plunge, decided to register and join in on the discussion.

I am using the book "Practical C Programming" to help teach myself how to program in C. I've gone through the beginning chapters and done one of the exercises. I was hoping I could get suggestions on how to improve on it or ways I could do things more efficiently. The exercise reads as follows:

Write a program to perform date arithmetic such as how many days there are between 6/6/90 and 4/3/92.

This is what I have so far:
Code:
#include <stdio.h>

int dayspassed(int month, int day, int year, int sMonth, int sDay, int sYear);

int main()
{
    char buffer[20] = {0};
    int day = 0, month = 0, year = 0;
    int sDay = 0, sMonth = 0, sYear = 0;

    printf("\nInput first date in MM/DD/YY format: ");
    fgets(buffer, sizeof(buffer), stdin);
    sscanf(buffer, "%d/%d/%d", &month, &day, &year);

    if ((month > 12) || (day > 31))
        printf("Error: Invalid date.\n");
    else
        printf("You chose %d/%d/%d.\n", month, day, year);

    printf("\nInput second date in MM/DD/YY format: ");
    fgets(buffer, sizeof(buffer), stdin);
    sscanf(buffer, "%d/%d/%d", &sMonth, &sDay, &sYear);

    if ((sMonth > 12) || (sDay > 31))
        printf("Error: Invalid date.\n");
    else
        printf("You chose %d/%d/%d.\n", sMonth, sDay, sYear);

    printf("The number of days between %d/%d/%d and %d/%d/%d is %d.",
    month, day, year, sMonth, sDay, sYear, dayspassed(month, day, year, sMonth, sDay, sYear));

    return (0);
}

int dayspassed(int month, int day, int year, int sMonth, int sDay, int sYear)
{
    int diffDay = 0, diffMonth = 0, diffYear = 0, days = 0;

    diffDay = sDay - day;
    diffMonth = sMonth - month;
    diffYear = sYear - year;

    days = diffDay + (diffMonth * 31) + (diffYear * 365);

    return (days);
}
I'm mainly looking for information on some methods of input validation, but suggestions on features to add would be great also.

I also was wondering if there is any way to initialize all the variables in one line to 0. Take this piece of code for example:

Code:
int day = 0, month = 0, year = 0;
int sDay = 0, sMonth = 0, sYear = 0;
I figured you could just do day = month = year = 0 but that doesn't seem to work.

Thanks in advance and flame away if I did something stupid.