```ts
/** How many days lie between two proleptic Gregorian calendar dates. */

const MONTH_LENGTHS = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

function isLeap(year: number): boolean {
  return year % 4 === 0;
}

function monthLength(year: number, month: number): number {
  if (month === 2 && isLeap(year)) {
    return 29;
  }
  return MONTH_LENGTHS[month - 1];
}

function toOrdinal(date: number[]): number {
  const [year, month, day] = date;
  const prior = year - 1;
  let days = prior * 365;
  days += Math.floor(prior / 4);
  for (let m = 1; m < month; m += 1) {
    days += monthLength(year, m);
  }
  return days + day;
}

export function spanDays(start: number[], end: number[]): number {
  if (start.length !== 3 || end.length !== 3) {
    throw new Error("Invalid date list length");
  }
  if (start[0] > end[0]) {
    throw new Error("Start date after end date");
  }
  if (start[1] < 1 || start[1] > 12) {
    throw new Error("Month out of range");
  }
  if (start[2] < 1 || start[2] > monthLength(start[0], start[1])) {
    throw new Error("Day out of range");
  }
  if (end[1] < 1 || end[1] > 12) {
    throw new Error("Month out of range");
  }
  if (end[2] < 1 || end[2] > monthLength(end[0], end[1])) {
    throw new Error("Day out of range");
  }
  return toOrdinal(end) - toOrdinal(start);
}
```