Find out `a-b-c` is `u-v-w` after `x-y-z` (`yyyy-MM-dd`).

pretty-date-diff.c

/* https://www.v2ex.com/t/879372 */

int days_in_month(int m)
{
  switch (m % 12)
  {
    case 0: case 2: case 4: case 6: case 7: case 9: case 11:
      return 31;
    case 3: case 5: case 8: case 10:
      return 30;
    case 2: default:
      m /= 12;
      return m % 3200 == 0 ? 28
        : m % 400 == 0 ? 29
        : m % 100 == 0 ? 28
        : m % 4 == 0 ? 29
        : 28;
  }
}

/* yyyy-MM-dd: a-b-c is u-v-w after x-y-z. */
void pretty_date_diff
(
  int a, int b, int c,
  int x, int y, int z,
  int *u, int *v, int *w
)
{
  /* Error checks are elided. */
  int aligned_month = a * 12 + (b - 1);
  int days = c - z;
  if (days < 0)
  {
    --aligned_month;
    days += days_in_month(aligned_month);
  }
  /* "days < 0" is still possible here, but
  /* if so, the following branch will be taken. */
  if (z > days_in_month(aligned_month))
  {
    --aligned_month;
    z += 31;
  }
  int months_passed = aligned_month - x * 12 - (y - 1);
  *u = months_passed / 12;
  *v = months_passed % 12;
  *w = days;
}