Adjust months and years based on the days overflow

example/lib/main.dart

@@ -33,9 +33,32 @@ void main() {
   // You next birthday will be in Years: 4, Months: 4, Days: 0
 
   // Add time to any date
-  DateTime date = AgeCalculator.add(
+  DateTime date;
+
+  print('\nAdd Example:');
+  date = AgeCalculator.add(
       date: DateTime(2021, 1, 2),
       duration: DateDuration(years: 5, months: 2, days: 1));
   print(date);
   // 2026-03-03 00:00:00.000
+
+  date = AgeCalculator.add(
+      date: DateTime(2023, 2, 28),
+      duration: DateDuration(months: 6));
+  print(date);
+  // 2024-02-29 00:00:00.000
+
+  // Subtract time to any date
+  print('\nSubtract Example:');
+  date = AgeCalculator.subtract(
+      date: DateTime(2021, 1, 2),
+      duration: DateDuration(years: 5, months: 2, days: 1));
+  print(date);
+  // 2015-11-01 00:00:00.000
+
+  date = AgeCalculator.subtract(
+      date: DateTime(2023, 8, 31),
+      duration: DateDuration(months: 6));
+  print(date);
+  // 2023-02-28 00:00:00.000
 }

lib/age_calculator.dart

@@ -1,5 +1,7 @@
 library age_calculator;
 
+import 'dart:math';
+
 class DateDuration {
   int days;
   int months;
@@ -95,13 +97,19 @@ class AgeCalculator {
   /// add method
   static DateTime add(
       {required DateTime date, required DateDuration duration}) {
-    int years = date.year + duration.years;
-    years += ((date.month + duration.months) ~/ DateTime.monthsPerYear);
-    int months = ((date.month + duration.months) % DateTime.monthsPerYear);
-
-    int days = date.day + duration.days - 1;
+    var newDateTime = date.add(Duration(days: duration.days));
+    newDateTime = _addMonths(newDateTime, duration.months);
+    newDateTime = _addMonths(newDateTime, duration.years * 12);
+    return newDateTime;
+  }
 
-    return DateTime(years, months, 1).add(Duration(days: days));
+  /// subtract methods
+  static DateTime subtract(
+      {required DateTime date, required DateDuration duration}) {
+    var newDateTime = date.subtract(Duration(days: duration.days));
+    newDateTime = _addMonths(newDateTime, -duration.months);
+    newDateTime = _addMonths(newDateTime, -duration.years * 12);
+    return newDateTime;
   }
 
   static DateDuration age(DateTime birthdate, {DateTime? today}) {
@@ -117,4 +125,16 @@ class AgeCalculator {
         : tempDate;
     return dateDifference(fromDate: endDate, toDate: nextBirthdayDate);
   }
+
+  static DateTime _addMonths(DateTime dateTime, int months) {
+    final modMonths = months % 12;
+    var newYear = dateTime.year + ((months - modMonths) ~/ 12);
+    var newMonth = dateTime.month + modMonths;
+    if (newMonth > 12) {
+      newYear++;
+      newMonth -= 12;
+    }
+    final newDay = min(dateTime.day, daysInMonth(newYear, newMonth));
+    return DateTime(newYear, newMonth, newDay);
+  }
 }