Maman 12 Q5 in C

#include <stdio.h>

// reads till the first non-digit char
int strToInt(char *a0) {
    // find number of digits
    unsigned int t1=0, t2=1, t3=0, t4=0, t5=0;
    t3 = *(a0+t1);
    while ( t3 <= '9' && t3 >= '0' ) {
        t1++;
        t3 = *(a0+t1);
    }
    t1--; // adjust to be an index
    while ( t1 != -1 ) {
        t3 = *(a0+t1);
        t3 -= '0';
        t4 = t3 * t2;
        t5 += t4;
        t1--;
        t2 *= 10;
    }
    return t5;
}

int main(void) {
    char *date = "23.11.2012";
    char buf[14];
    int stop = 0x706f7473; // bytes are reversed
    int s1, s2, s3, s4, s5, s6, t1, t2;

    printf("%s\n", date);

    s1 = strToInt(date);
    s2 = strToInt(date+3);
    s3 = strToInt(date+6);
    // assume each month has 31 days and each year 356
    t1 = s1;
    t1 += ( s2 * 31 ) + ( s3 * 356 );
    while ( fgets(buf, 12, stdin) != NULL ) {
        // check for stop
        if ( *(int*)buf == stop )
            break;
        // check string correctness
        s4 = strToInt(buf);
        s5 = strToInt(buf+3);
        s6 = strToInt(buf+6);

        // check that the dates are valid
        if ( s4 <= 1 || s4 >= 31 || s5 <= 1 || s5 >= 12 || s6 >= 5000 )
            goto error;
        // assume each month has 31 days and each year 356
        t2 = s4;
        t2 += ( s5 * 31 ) + ( s6 * 356 );
        if ( t2 <= t1 )
            printf("o.k.\n");
        else
            printf("too late!\n");
    }

    return 0;
    error:
        printf("error\n");
    return 1;
}