Add C solution

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+/*
+Challenge 038
+
+TASK #1
+Date Finder
+Create a script to accept a 7 digits number, where the first number can only
+be 1 or 2. The second and third digits can be anything 0-9. The fourth and
+fifth digits corresponds to the month i.e. 01,02,03…,11,12. And the last
+2 digits respresents the days in the month i.e. 01,02,03….29,30,31. Your
+script should validate if the given number is valid as per the rule and then
+convert into human readable format date.
+
+RULES
+If 1st digit is 1, then prepend 20 otherwise 19 to the 2nd and 3rd digits to
+make it 4-digits year.
+
+The 4th and 5th digits together should be a valid month.
+
+The 6th and 7th digits together should be a valid day for the above month.
+
+For example, the given number is 2230120, it should print 1923-01-20.
+*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+bool is_leap_year(int y) {
+    return y % 400 == 0 ? true : y % 100 == 0 ? false : y % 4 == 0 ? true : false;
+}
+
+int days_of_month(int y, int m) {
+    switch (m) {
+    case 1: case 3: case 5: case 7: case 8: case 10: case 12: return 31;
+    case 4: case 6: case 9: case 11: return 30;
+    case 2: return is_leap_year(y) ? 29 : 28;
+    default: assert(0); return -1;
+    }
+}
+
+bool decode_date(int code, int* y, int* m, int* d) {
+    *d = code % 100; code /= 100;
+    if (*d < 1 || *d > 31) return false;
+
+    *m = code % 100; code /= 100;
+    if (*m < 1 || *m > 12) return false;
+
+    int year_lsb = code % 100; code /= 100;
+    int year_msb = code;
+    if (year_msb != 1 && year_msb != 2) return false;
+
+    *y = (year_msb == 1 ? 2000 : 1900) + year_lsb;
+
+    if (*d > days_of_month(*y, *m)) return false;
+
+    return true;
+}
+
+int main(int argc, char* argv[]) {
+    if (argc != 2) {
+        fputs("usage: ch-1 date", stderr);
+        return EXIT_FAILURE;
+    }
+
+    int date_code = atoi(argv[1]);
+    int y, m, d;
+    if (!decode_date(date_code, &y, &m, &d)) {
+        fputs("invalid date", stderr);
+        return EXIT_FAILURE;
+    }
+
+    printf("%04d-%02d-%02d\n", y, m, d);
+}