您可以根据需要定制以下代码，以返回字符串或对象，或使其成为扩展：

void getDiffYMD(DateTime then, DateTime now) {
  int years = now.year - then.year;
  int months = now.month - then.month;
  int days = now.day - then.day;
  if (months < 0 || (months == 0 && days < 0)) {
    years--;
    months += (days < 0 ? 11 : 12);
  }
  if (days < 0) {
    final monthAgo = DateTime(now.year, now.month - 1, then.day);
    days = now.difference(monthAgo).inDays + 1;
  }

  print('$years years $months months $days days');
}

答案可能有点长，但会给予你你需要的。
这将以字符串形式给予值，格式为“Year：年、月：月，天：天”。您可以根据需要更改。
您需要调用differenceInYearsMonthsAndDays（）并传递开始日期和结束日期。
一个扩展，用于计算以月为单位的差值，并丢弃多余的天数。

extension DateTimeUtils on DateTime {
  int differenceInMonths(DateTime other) {
    if (isAfter(other)) {
      if (year > other.year) {
        if (day >= other.day) {
          return (12 + month) - other.month;
        } else {
          return (12 + month - 1) - other.month;
        }
      } else {
        if (day >= other.day) {
          return month - other.month;
        } else {
          return month - 1 - other.month;
        }
      }
    } else {
      return 0;
    }
  }
}

一种生成年、月、日格式的方法

String differenceInYearsMonthsAndDays(
  DateTime startDate,
  DateTime endDate,
) {
  int days;
  final newStartDate = startDate.add(const Duration(days: -1));
  int months = endDate.differenceInMonths(newStartDate);
  if(months >= 12) {
    final years = months ~/ 12;
    final differenceInMonthsAndDays = differenceInMonthsAndDays(
          startDate.add(const Duration(year: years), 
          endDate,
    );
    return "Years : years, $differenceInMonthsAndDays";
  } else {
    return differenceInMonthsAndDays(
          startDate, 
          endDate,
    );
  }

}

一种生成月、日格式的方法

String differenceInMonthsAndDays(
  DateTime startDate,
  DateTime endDate,
) {
  int days;
  final newStartDate = startDate.add(const Duration(days: -1));
  int months = endDate.differenceInMonths(newStartDate);

  if (months > 0) {
    final tempDate = DateTime(
      newStartDate.year,
      newStartDate.month + months,
      newStartDate.day,
    );
    days = endDate.difference(tempDate).inDays;
    return days > 0 
       ? "Months : $months, Days : $days" 
       : "Months : $months";
  } else {
    days = endDate.difference(newStartDate).inDays;
    return "Days : $days";
  }
}