isSameDay method
Checks if two DateTime instances represent the same calendar day.
Example usage:
final date1 = DateTime(2020, 1, 1);
final date2 = DateTime(2020, 1, 1, 12, 0, 0);
print(date1.isSameDay(date2)); // true
Implementation
bool isSameDay(DateTime b) => year == b.year && month == b.month && day == b.day;