getDaysSpanned method

Implementation

int getDaysSpanned(DateTime rangeStart, DateTime rangeEnd) {
  // Helper to normalize a date to midnight UTC.
  DateTime normalize(DateTime dt) => DateTime.utc(dt.year, dt.month, dt.day);

  final effectiveStart =
      startDate.isAfter(rangeStart) ? startDate : rangeStart;
  var effectiveEnd = endDate.isBefore(rangeEnd) ? endDate : rangeEnd;

  // If an event's end time is exactly midnight, it concludes at the very beginning of that day,
  // meaning it doesn't occupy that day's calendar slot. So we calculate the difference directly.
  // The exception is a single-day event, which should always have a span of at least 1.
  if (!isSameDay(effectiveStart, effectiveEnd) &&
      effectiveEnd.hour == 0 &&
      effectiveEnd.minute == 0 &&
      effectiveEnd.second == 0 &&
      effectiveEnd.millisecond == 0 &&
      effectiveEnd.microsecond == 0) {
    return normalize(effectiveEnd)
        .difference(normalize(effectiveStart))
        .inDays;
  }

  return normalize(effectiveEnd)
          .difference(normalize(effectiveStart))
          .inDays +
      1;
}