C program to convert string to integer without using atoi function

#include <stdio.h>
#include <string.h>

long long mypow(int m){
    long long di = 1;
    if(m == 0){
        return 1;
    }
    for(int i = 0; i<m; i++){
        di = di * 10;
    }
    return di;
}
long long toint64(const char *src){
    long long n = 0;
    int m = 0;
    int len = strlen(src) - 1;
    while(len + 1){
        if((src[len] <= '9') && (src[len] >= '0')){
            n += (src[len] -'0') * (mypow(m));
            m++;
        }
        len--;
    }
    return n;
}

int main()
{
    char *a = "2019-12-04 00:00;";
    char *b = "1a2b3c4d5**678U 9";
    printf("%I64d\n",toint64(a));
    printf("%I64d\n",toint64(b));
    return 0;
}