contains method
Returns true if date falls within this range (inclusive of both ends).
Implementation
bool contains(DateTime date) {
final dateOnly = DateTime(date.year, date.month, date.day);
final startOnly = DateTime(start.year, start.month, start.day);
final endOnly = DateTime(end.year, end.month, end.day);
return !dateOnly.isBefore(startOnly) && !dateOnly.isAfter(endOnly);
}