```python
"""How many days lie between two proleptic Gregorian calendar dates."""

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


def _is_leap(year):
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)


def _month_length(year, month):
    if month == 2 and _is_leap(year):
        return 29
    return MONTH_LENGTHS[month - 1]


def _to_ordinal(date):
    year, month, day = date
    if not all(isinstance(x, int) for x in date):
        raise ValueError("All date components must be integers")
    if year < 1 or month < 1 or month > 12 or day < 1 or day > _month_length(year, month):
        raise ValueError("Invalid date")
    prior = year - 1
    days = prior * 365 + prior // 4 - prior // 100 + prior // 400
    for earlier in range(1, month):
        days += _month_length(year, earlier)
    return days + day


def span_days(start: list, end: list) -> int:
    if start > end:
        raise ValueError("Start date must not be after end date")
    return _to_ordinal(end) - _to_ordinal(start)
```