An implementation of formatDuration in Dart, probably needs optimizations but it works and has support for changing the strings; getDateDifference by @app-o-matix

format_duration.dart

import 'package:collection/collection.dart';

Function listEquals = const ListEquality().equals;

class DateUtils {
 
  static String formatDuration(DateTime dateTime,
      {String years = "year(s)",
      String months = "month(s)",
      String days = "day(s)",
      String and = "and",}) {
    final difference = getDateDifference(dateTime, DateTime.now());

    final Map<int, int> valid = {};
    for (var i = 0; i < difference.length; i++) {
      valid[i] = difference[i];
    }
    valid.removeWhere((key, value) => value == 0);

    if (valid.length == difference.length)
      return "${valid[0]} $years, ${valid[1]} $months $and ${valid[2]} $days";
    else {
      final List<int> keys = valid.keys.toList();
      if (listEquals(keys, [0, 1]))
        return "${valid[0]} $years $and ${valid[1]} $months";
      else if (listEquals(keys, [0, 2]))
        return "${valid[0]} $years $and ${valid[2]} $days";
      else if (listEquals(keys, [1, 2]))
        return "${valid[1]} $months $and ${valid[2]} $days";
      else if (listEquals(keys, [0]))
        return "${valid[0]} $years";
      else if (listEquals(keys, [1]))
        return "${valid[1]} $months";
      else if (listEquals(keys, [2]))
        return "${valid[2]} $days";
      else
        return "";
    }
  }

  static List<int> getDateDifference(
      DateTime initialDate, DateTime subsequentDate) {
    // Convert initialDate DateTime value to a a List<int> represent [year, month, day]
    List<int> _initialDateSplit = [
      initialDate.year,
      initialDate.month,
      initialDate.day
    ];

    // Convert subsequentDate DateTime value to a a List<int> represent [year, month, day]
    List<int> _subsequentDateSplit = [
      subsequentDate.year,
      subsequentDate.month,
      subsequentDate.day
    ];

    List<int> _durationBetween = [];

    print(
        "Initial Date - year: ${_initialDateSplit[0]}, month: ${_initialDateSplit[1]}, day: ${_initialDateSplit[2]}");
    print(
        "Subsequent Date - year: ${_subsequentDateSplit[0]}, month: ${_subsequentDateSplit[1]}, day: ${_subsequentDateSplit[2]}");

    // Calculate year difference
    _durationBetween.add(_subsequentDateSplit[0] - _initialDateSplit[0]);

    // Calculate month difference
    if (_initialDateSplit[1] <= _subsequentDateSplit[1]) {
      _durationBetween.add(_subsequentDateSplit[1] - _initialDateSplit[1]);
      // Adjust year difference if initial month is after subsequent month in the calendar
    } else {
      _durationBetween.add(12 - _initialDateSplit[1] + _subsequentDateSplit[1]);
      --_durationBetween[0];
    }

    // Calculate day difference
    if (_initialDateSplit[2] <= _subsequentDateSplit[2]) {
      _durationBetween.add(_subsequentDateSplit[2] - _initialDateSplit[2]);
      // Adjust month difference if initial day is after subsequent day as a number
      // and the initial month is a 31 day month
    } else if ([1, 3, 5, 7, 8, 10, 12].contains(_initialDateSplit[1])) {
      _durationBetween.add(31 - _initialDateSplit[2] + _subsequentDateSplit[2]);
      --_durationBetween[1];
      // Adjust month difference if initial day is after subsequent day as a number
      // and the initial month is a 30 day month
    } else if ([4, 6, 9, 11].contains(_initialDateSplit[1])) {
      _durationBetween.add(30 - _initialDateSplit[2] + _subsequentDateSplit[2]);
      --_durationBetween[1];
      // Adjust month difference if initial day is after subsequent day as a number
      // and the initial month is February and the initial year is a leap year
    } else if ((_initialDateSplit[0] % 4 == 0 &&
            _initialDateSplit[0] % 100 != 0) ||
        _initialDateSplit[0] % 400 == 0) {
      _durationBetween.add(29 - _initialDateSplit[2] + _subsequentDateSplit[2]);
      --_durationBetween[1];
      // Adjust month difference if initial day is after subsequent day as a number
      // and the initial month is February and the initial year is not a leap year
    } else {
      _durationBetween.add(28 - _initialDateSplit[2] + _subsequentDateSplit[2]);
      --_durationBetween[1];
    }

    return _durationBetween.toList();
  }
  
}